UNIVERSITY  OF  CALIFORNIA 

ANDREW 

SMITH 

HALLIDIE: 


SHOP  AND  ROAD  TESTING 

OF 

DYNAMOS   AND   MOTORS 

A  PRACTICAL   MANUAL  FOR    THE  TESTING 
FLOOR,    THE  CAR  BARN  AND  THE  ROAD. 

BY 

EUGENE  C.    PARHAM,   M.  E. 

1 1 

Formerly  Electrician  Steel  Motor  Co.,   Johnstown,  Pa. 
AND 

JOHN   C.   SHEDD,   PH.  D. 

Professor  of  Physics,   Colorado  College,  formerly  Fellow  and 

Instructor  in  the   University  of  Wisconsin  and 

Professor  of  Physics  at  Marietta  College. 


Experience  may  bt  a  dear  tck*oly  but  it  it  the  best. 


NEW  YORK 

McGRAW  PUBLISHING  CO. 
1901 


COPYRIGHT,   1901, 

ELECTRICAL    WORLD    AND    ENGINEER, 
(INCORPORATED.) 


PREFACE. 


THE  following  chapters  are  the  outcome  of  a  need  felt 
by  the  authors,  during  their  experience  on  the  testing  floor 
and  road,  for  a  manual  that  would  cover  this  field  of  work 
in  such  a  way  as  to  be  a  help  alike  to  the  student  fresh 
from  the  theoretical  side  of  the  subject  but  unacquainted 
with  shop  details,  and  to  the  so-called  practical  man,  who 
is  largely  self-taught,  as  to  the  theory  of  the  machines  he 
handles. 

On  the  purely  theoretical  side  such  works  as  those  of 
S.  P.  Thompson  and  D.  C.  Jackson  leave  little  to  be 
desired,  while  from  the  purely  practical  side  a  multitude 
of  so-called  practical  books  flood  the  market;  but  works 
that  are  at  once  correct  from  the  standpoint  of  the 
theorist,  and  yet  valuable  to  the  strictly  practical  man, 
have  not  been  easy  to  find.  This  need  is  felt  by  the 
college  graduate  as  he  steps  from  the  plane  of  the 
laboratory  and  lecture  room  to  the  hard-pan  of  the  com- 
mercial testing  floor.  It  is  also  felt  by  a  multitude  of 
station  managers  and  engineers  who  find  the  hit-and-miss 
methods  of  testing  far  from  satisfactory. 

The  present  work  has  a  two-fold  object:  I.  To  give 
a  complete  theory  of  the  commercial  testing  floor,  so  far 

iii 


UNIVERSITY 


IV  PREFACE. 

as  it  relates  to  direct  current  machines,  and  of  the 
multitudinous  applications  of  theory  to  practice.  II.  To 
meet  the  growing  demand  on  the  part  of  operating  com- 
panies for  a  manual  that  shall  enable  them  to  do  their 
own  repair  work  and  consequent  testing. 

Part  I.  is  devoted  to  such  fundamental  and  preliminary 
conceptions  as  are  needed  to  help  those  unacquainted 
with  the  general  theory.  It  may  be  omitted  by  many 
readers. 

Part  II.  treats  of  instrumental  testing.  The  treatment 
of  the  ammeter,  voltmeter,  and  galvanometer  is  mathe- 
matically simple  and  seeks  to  give  the  physical  con- 
ception embodied  in  the  formulae. 

Part  III.  takes  up  in  detail  the  tests  of  dynamos  and 
motors.  Special  attention  has  been  given  to  the  many 
difficulties  that  confront  the  tester,  and  all  examples  and 
illustrations  are  drawn  from  personal  experience.  The 
chapter  on  Compounding  is  specially  full.  It  is  hoped 
that  the  chapter  on  Grounds  on  the  Line  may  be  of 
service  to  the  lighting  station  and  street  railway 
operator. 

Care  has  been  observed  not  to  stray  from  the  paths  of 
actual  practice  on  the  one  hand,  and,  on  the  other,  not  to 
offer  inadequate  or  incorrect  explanations. 

From  the  nature  of  the  case  mathematical  treatment 
has  been  simplified  to  the  last  degree,  and  even  the 
graphical  method  but  little  used. 

The  International  electrical  units  have  been  adopted  and 
the  following  abbreviations  used:  Electromotive  force, 


PREFACE.  V 

E.  M.  F.;  counter  electromotive  force,  C.  E.  M.  F.; 
alternating  current  electromotive  force,  A.  C.  E.  J/.  P.; 
current,  /,  /;  resistance,  R,  r;  other  abbreviations  that 
have  been  used  are  self-explanatory. 

The  authors  take  pleasure  in  acknowledging  the 
courtesy  and  help  they  have  received  from  several  of 
the  manufacturing  companies  and  from  old  associates 

and  friends. 

EUGENE  C.  P  ARM  AM, 

JOHN  C.  SHEDD. 
OCTOBER,  1897. 


PREFACE  TO  SECOND  EDITION. 


In  meeting  the  demand  for  a  second  edition  of  this 
work,  the  authors  have  taken  the  opportunity  of  cor- 
recting such  typographical  errors  as  have  come  to  their 
attention.  They  have  also  extended  the  scope  of  the 
book  so  as  to  include  the  field  of  street-car  equipment 
and  operation.  In  this  the  endeavor  has  been  to  be 
strictly  practical  in  the  selection  and  treatment  of  the 
subjects  discussed.  At  the  same  lime  it  is  the  aim  of  the 
writers  to  aid  the  reader  in  gaining  a  comprehensive  grasp 
of  the  principles  involved  in  the  problems  discussed,  and 
to  thereby  enable  him  to  successfully  meet  the  numerous 
difficulties  which  the  man  on  the  front  platform  con- 
tinually encounters. 

It  is  believed  that  Part  IV  will  be  found  a  valuable 
addition  to  the  book,  and  that  the  high  standard  aimed 
at  in  the  balance  of  the  work  is  fully  maintained. 

E.  C.  PARHAM, 
JOHN  C.   SHEDD. 


CONTENTS. 


INTRODUCTION. 
CHAPTER  I. 

ELEMENTS   OF   THE    DYNAMO. 

PAGE 

Laws  of  Energy ;  Units  of  Electricity  and  Magnetism  ; 
Permeability  ;  the  Magnetic  Circuit ;  the  Ampere,  Volt, 
Ohm  ;  Induction  ;  Magnetomotive  Force  ;  Electromag- 
netic Induction  ;  Elements  of  the  Dynamo  ;  Methods 
of  Excitation ;  Series,  Shunt,  and  Compound-Wound 
Machines ;  Losses  in  the  Dynamo ;  Cross  and  Back 
Induction  ;  Efficiency, 3 


CHAPTER   II. 

ELEMENTS    OF    THE    MOTOR. 

The  Electric  Motor  ;  Counter  Electromotive  Force  ;  Elec- 
trical and  Commercial  Efficiency  ;  Maximum  Activity  ; 
Maximum  Efficiency ;  Torque  ;  Speed  Regulation  ; 
Classification  of  Motors  ;  Series,  Shunt,  and  Compound- 
Wound  Motors ;  Direction  of  Rotation ;  Motors  and 
Dynamos, 36 

vii 


Vlll  CONTENTS. 

THE  TESTING  AND  USE   OF  INSTRUMENTS. 

CHAPTER   III. 

OHM'S  LAW. 

PAGE 

General  Discussion  of  Ohm's  Law ;  Resistance  ;  Conduc- 
tivity ;  Fall  of  Potential  ;  Various  Expressions  for  Ohm's 
Law  ;  Joule's  Law  ;  Inductive  Circuits  ;  Further  Defi- 
nition of  Resistance,  59 

CHAPTER   IV. 

MEASUREMENT   OF   CURRENT, 

Measurement  of  Current  by  Copper  Voltameter  ;  Standard- 
izing Ammeters  by  the  Voltameter ;  Fundamental 
Principles  of  the  Galvanometer  ;  Determination  and 
Explanation  of  Galvanometer  Constants  ;  Theory  and 
Application  of  Shunts  ;  Construction  of  Standard  Shunt  ; 
Its  Use  with  Ammeters  ;  Multiple  Circuits  ;  Siemens 
Dynamometer ;  Wattmeter, 70 


CHAPTER  V. 

MEASUREMENT   OF   ELECTROMOTIVE   FORCE. 

Definition  of  E.  M.  F.  and  P.  D.;  Standards  of  E.  M.  F.; 
General  Discussions  of  Batteries  ;  Daniell's  Standard 
Cell ;  Leclanche  Cell  ;  the  Galvanometer,  its  Construc- 
tion, Adjustment,  and  Mounting ;  Measurement  of 
E.  M.  F.  by  the  Galvanometer ;  Proportion  Lines  ;  Pro- 
portion Boxes  ;  Wiring  the  Galvanometer  ;  Clark  Cell ; 
Calibrating  a  Voltmeter  ;  Slide  Wire  Method  of  Measur- 
ing E.  M.  F.;  Multipliers, 112 


CONTENTS.  IX 

CHAPTER   VI. 

MEASUREMENT   OF    RESISTANCE. 

PAGE 

Resistance  in  General  ;  Measurement  of  Low  Resistance  ; 
Method  of  Comparison  of  Potentials  ;  Locating  Armature 
Faults ;  Locating  Faulty  Section  ;  Bar  to  Bar  Test  ; 
Grounds  on  Armatures ;  Measurement  of  Moderate 
Resistances  ;  "  Vienna"  Method  ;  Theory  of  Wheatstone 
Bridge,  Slide  Bridge,  Box  Bridge,  Portable  Bridge, — 
Their  Application  ;  Very  Low  Resistance  ;  Sir  W.Thom- 
son's Bridge  ;  Remarks  on  Bridge  Work  ;  Differential 
Galvanometer,  Its  Theory  and  Practice,  .  .  .145 

CHAPTER   VII. 

MEASUREMENT    OK    INSULATION. 

Insulation  Measurement  with  Galvanometer  and  Shunt  Box  ; 
Testing  Glass  Insulators ;  Marine  and  Underground 
Cables ;  Telephone  Cables ;  Electrometer  Method  ; 
Armature  and  Field  Insulation  ;  General  Remarks  on 
Insulation  Work  ;  Alternating  Current  Test  for  High 
Tension  Service  ;  Liquid  Resistances  ;  Battery  Resist- 
ances, Water  Rheostats  ;  Slide  Bridge  and  Telephone 
Method  ;  Temperature  Coefficient  of  Resistance  ;  Specific 
Resistance  ;  General  Remarks  on  Instruments,  .  .183 

TESTING   OF    DYNAMOS   AND    MOTORS. 
CHAPTER  VIII. 

THE  SERIES    MACHINE. 

General  Discussion  ;  Troubles  Incident  to  Self-Exciting 
Machines  ;  Tests  for  Open  and  Short  Circuits  in  Field 
and  Armature  ;  Tests  for  Grounded  Armature  ;  Run- 
ning Series  Machines  in  Multiple;  Regulation  in  Arc 
Dynamos ;  the  Thomson-Houston  Arc  Dynamo ;  Brush 
Arc  Dynamo  ;  the  Westinghouse  Arc  Dynamo,  .  .  231 


X  CONTENTS. 

CHAPTER   IX. 

SHUNT   AND    COMPOUND-WOUND    MACHINE. 

PAGE 

Shunt  Machine  ;  General  Considerations  ;  E.  M.  F.  Regula- 
tion ;  Field  Resistance  ;  Efficiency  ;  Rheostat  Regula- 
tion ;  Effect  of  Temperature  ;  Limits  of  Regulation  ; 
Rheostats— Parallel  and  Series  Running  ;  Putting  a 
Machine  into  Circuit;  Direction  of  Rotation  upon  Rever- 
sal of  Shunt  and  Series  Machines  C.  W.  Machines;  Con- 
nections for  Parallel  Working  ;  Introducing  and  Taking 
a  Machine  from  Service  ;  Principles  of  Compounding  ; 
Over-Compounding;  Compounding  Volts  per  Revolution; 
Compounding  a  Shunt  Machine  without  Instruments,  .  279 

CHAPTER    X. 

THE   COMPOUND-WOUND     MACHINE — GENERAL   TESTS. 

The  Equalizing  Bar  ;  Multiple  and  Series  Running  ;  Test  I. 
Test  of  an  Eight-Volt  Twenty-Five-Ampere  Shunt 
Machine  ;  Test  II.  Motor-Generator  Test  with  Engine 
as  Loss-Supplier  ;  Test  III.  Motor-Generator  Test  with 
Lamp-Bank;  Test  IV.  Motor  Generator  Test  with  three 
Machines  ;  Test  V.  Motor-Generator  Test,  Machines  of 
Different-Current  Capacity  ;  Test  VI.  Motor-Generator 
Test,  Same  as  Test  V.  with  Lamp-Bank  ;  Test  VII. 
Motor-Generator  Test,  Same  as  Test  V.  with  Motor  ; 
Test  VIII.  Motor-Generator  Test,  Machines  of  Different 
E.  M.  F.  and  Current  Capacity  ;  Test  IX.  Test  of  Single 
Machine,  with  Water  Rheostat 331 

CHAPTER   XL 

COMPOUNDNG. 

Test  X.  Compounding  Test  with  Full  Details,  to  Run 
Under  Full  Load  Two  500  Volt  500 K.W.  Railway  Gener- 
ators and  to  Supply  the  Loss  from  an  Auxiliary  Dynamo 
of  the  Same  Voltage  ;  Test  XI.  Compounding  Test, 
Three  Machines  Run  in  Series, 378 


CONTENTS.  XI 

CHAPTER   XII. 

MISCELLANEOUS    TESTS. 

PAGE 

Test  XII.  Core-loss  Test ;  Test  XIII.  Eddy  Current  Test ; 
Test  XIV.  Saturation  Test;  Test  XV.  Distribution  Test; 
Test  XVI.  Brush  Test;  Test  XVII.  Efficiency  Test,  .  419 

CHAPTER   XIII. 

GROUNDS    ON   THE    LINE. 

The  Ground  Detector  ;  Use  of  Voltmeter  in  Measuring  Leak- 
age Resistance;  Use  of  the  Ammeter  in  Measuring  Resist- 
ance of  a  Fault ;  Use  of  Voltmeter  for  the  Same  ;  Ground 
Detector  on  Alternating  Current  Circuits  ;  Picou's  Con- 
densor  Method  for  Alternating  Current  Circuits  ;  Stanley 
Static  Ground  Detector  ;  Methods  of  Locating  Grounds  ; 
Magneto  Method ;  Underground  Circuits  and  Fall  of 
Potential  Method  ;  First  Bridge  Method  ;  Second  Bridge 
Method  ;  Burning  out  Faults 439 

CHAPTER  XIV. 

MOTOR    TESTING. 

Classification  of  Motors  ;  Cross-connecting  of  Armatures  and 
Commutators ;  Starting  Boxes  for  Shunt  and  Series 
Motors ;  Tendency  of  Series  Motors  to  Race  under  no 
Load  ;  Speed  under  Sudden  Removal  of  Load  ;  Test 
XVIII.  Motor  Generator  Test  with  Street  Car  Motors; 
Test  XIX.  Testing  Series  Machines  on  Water  Box  ;  Test 
XX.  Efficiency  Test ;  Test  XXL  Efficiency  Test  with 
Prony  Brake ;  Testing  Series  Machines  Rigidly  Con- 
nected ;  The  Compound- Wound  Motor,  in  General ;  Motor 
Work  in  Shops,  Speed,  Voltage,  Load  ;  Small  Motors ; 
Motors  Run  in  Series  ;  Stray  Facts  ;  Dynamos  and  Motor 
Rating 468 


Xll  CONTENTS. 

CHAPTER  XV. 

INSTALLATION   CAR   EQUIPMENT   TESTS. 

General  Remarks.  The  Trolley  Stand;  Setting  up,  Inspecting; 
Testing  of  Overhead  Switches  and  Circuit  Breakers;  Test- 
ing Fuse  Boxes  ;  Testing  and  Selecting  Fuses  ;  The 
Lightning  Arrester;  Kicking  Coil— Requirements,  Faults, 
Testing;  Starting  Coil — Use  and  Abuse  of,  Faults,  Testing; 
Shunt  and  Loop — Action  and  Object  of;  Symptoms  of 
Disorder;  Controllers;  Type  K2,  Connections,  Care  of; 
Motors — 50  H.P.  Modern  Motor,  Unique  Features  of.  .  510 

CHAPTER  XVI. 

CAR   EQUIPMENT   TESTS. 

The  Test  Circuit;  Construction  of  a  Test  Lamp  Bank;  Test- 
ing a  Starting  Coil  for — Open  Circuits,  Short  Circuits, 
Grounds;  Testing  a  Controller  for — Open  Circuits,  Short 
Circuits,  Grounds;  Testing  a  Motor  for — Open  Circuit  in 
Fields  and  Armature,  for  Polarity;  Marking  Motor  Leads; 
Setting  the  Gear;  Mounting  Motors,  General  Suggestions  as 
to  Care  and  Precautions;  Testing  the  Gears,  Symptoms  of 
Disorder  and  their  Causes;  Testing  Connections  by  Start- 
ing Up,  Faults  Located  by  Symptoms;  Grounds  and  Short 
Circuits;  Location  of  by  Test  Lines;  Voltmeter  Tests; 
Baked  or  Short  Circuited  Fields;  Reversed  Coils;  Test  with 
Nail  or  Compass;  Grounds  on  Armature,  Peculiar  Actions 
Resulting;  Open  Circuit  in  Armature;  Motors  used  as 
Electric  Brakes;  Bucking  of  Motors,  .....  558 

APPENDIX. 

Table       I.  Data  for  Copper  Wire, 613 

"         II.  Temperature  Coefficients  for  Copper,          .         .  614 

III.  Data  for  Galvanized  Iron  Wire,     .         .         .  .615 

"        IV.  Current  Capacity  for  Iron  Wire,         .         .         .  616 

"         V.  Fusing  Effects  of  Currents,     .         .         .         .  .617 

"*       VI.  Fusing  Effects  of  Currents,        ...        .  618 

"      VII.  Test  Sheet  for  Compounding,         .     '    .         .  .     619 

"   VIII.  Test  Sheet  for  Compounding 620 


SHOP  AND  ROAD   TESTING  OF 
DYNAMOS  AND  MOTORS 


INTRODUCTION 


CHAPTER  I. 

ELEMENTS    OF    THE    DYNAMO. 

WHEN  told  for  the  first  time  that  more  energy  is 
required  to  rotate  a  metal  disc  between  magnet  poles 
than  away  from  them,  the  beginner's  impulse  is  to  ques- 
tion the  statement;  if  further  told  that  the  disc  will 
grow  warm  under  continued  rotation,  his  interest  is 
aroused;  and  the  further  assertion  that  with  a  brush  on 
the  axle  and  another  on  the  rim,  the  wheel  can  be  made 
to  generate  an  electric  current,  calls  forth  his  demand 
for  experimental  proof. 

To  locate  the  true  seat  of  energy  in  a  dynamo  or 
motor  is  a  puzzling  problem  unsolved  in  the  minds  of 
many  electrical  and  mechanical  artisans.  The  intelligent 
workman  observes  the  belt  tugging  away  at  the  pulley, 
and,  falling  into  the  popular  misconception  of  the  case, 
he  imposes  the  burden  upon  every  sort  of  friction,  save 
that  of  the  magnetic  field,  to  which  it  is  due,  and  any 
explanation  involving  conductors,  lines  of  force,  and 


4  TESTING    OF    DYNAMOS    AND    MOTORS. 

kindred  terms,  evokes  a  look  of  bewilderment.  Nor  is 
this  strange.  Science  has  as  yet  given  no  fundamental 
answer  to  the  problem  of  the  metal  disc,  and  the 
transference  of  energy  without  the  intervention  of 
bodies  visibly  touching  each  other,  is  and  probably 
always  will  be,  a  source  of  wonder  to  the  wisest. 

In  the  above  simple  experiment  is  embodied  the 
problem  of  the  dynamo — that  of  converting  the  mechani- 
cal energy  of  the  engine  or  water  wheel  into  the  energy 
of  an  electric  current. 

Let  the  brushes  of  the  disc  be  connected  to  the  poles 
of  a  battery  and  the  conditions  of  the  problem  are 
reversed;  the  wheel  begins  to  turn  and  the  electrical 
energy  of  the  battery  is  transformed  into  energy  of 
mechanical  motion.  The  machine  is  now  an  electric 
motor.  From  this  experiment  of  Barlow  and  Faraday 
dates  the  beginning  of  all  practical  applications  of 
electrical  science. 

In  taking  up  any  scientific  study  it  is  necessary  to 
first  give  elementary  definitions  of  terms  and  phrases 
ordinarily  familiar,  and  to  acquaint  ourselves  with  the 
underlying  laws  of  the  science. 

There  are  two  things  which  can  be  said  to  exist  in  the 
physical  universe,  viz.,  Matter  and  Energy:  Matter,  as 
the  embodiment  of  all  tangible  things,  and  Energy  as  we 
can  appreciate  it  in  all  motion.  The  ordinary  idea  of 
matter  is  so  far  correct  as  to  require  no  further 
definition.  Energy  is  also  easily  recognized  and  may 
be  variously  defined.  The  flying  bullet  has  energy  and 
the  faster  the  motion  the  more  the  energy.  Energy, 
then,  is  due  to  motion  and  can  be  measured  in  terms  of  it. 
Again,  a  moving  body  strikes  a  second  body  at  rest,  and 


ELEMENTS    OF    THE    DYNAMO.  5 

the  latter,  if  free  to  move,  does  so,  while  the  former 
loses  part  of  its  motion.  The  second  body  now  has 
energy  in  virtue  of  its  motion,  and  we  observe  that 
energy  can  be  transferred  from  one  body  to  another. 
A  third  fact  is  gained  when  we  observe  that  if  a  moving 
body  is  stopped  by  an  obstacle,  as  a  hammer  by  the 
anvil,  both  bodies  become  heated,  and  the  greater  the 
energy  of  the  hammer  the  greater  the  heat  produced; 
and  further,  the  amount  of  heat  produced  is  an  exact 
measure  of  the  energy  expended,  or,  as  we  say,  of  the 
work  done.  This  brings  us  to  the  laws  of  energy  upon 
which  all  engineering  is  based,  and  a  thorough  under- 
standing of  which  would  save  many  an  industrious 
inventor  days  of  useless  toil.  The  first  law  is  that 
energy  is  indestructible  and  uncreatable  by  human 
agency.  By  this  law,  the  Law  of  the  Conservation  of 
Energy,  whenever  energy  seems  to  disappear  at  one 
point  we  must  look  for  it  elsewhere;  nor  rest  until  we 
find  its  exact  equivalent.  The  second  law  is  that  energy, 
while  indestructible,  may  be  transformed  into  many 
different  forms.  This  Law  of  Transformability  enables 
us  to  account  for  the  apparent  disappearance  of  energy. 
In  the  experiment  above,  to  turn  the  uninfluenced  disc 
requires  only  the  small  amount  of  energy  necessary  to 
overcome  the  friction  of  the  air  and  axle  boxes,  and, 
neglecting  the  former,  the  slight  expenditure  is  accounted 
for  by  the  heating  of  the  latter.  Upon  placing  the  mag- 
net over  the  disc,  as  in  Fig.  i,  more  energy  is  required  to 
keep  the  disc  in  motion,  more  energy  is  consumed,  and 
the  added  amount  is  accounted  for  by  the  presence  in 
the  disc  of  a  current,  between  which  and  the  magnet 
a  force  of  attraction  exists;  moreover,  by  the  first  law 


6  TESTING    OF    DYNAMOS    AND    MOTORS. 

the  amount  of  energy  lodged  in  the  current  will  depend 
upon  that  expended  in  turning  the  disc,  over  and  above 
that  l6st  in  frictional  heat. 

In  all   measurements   some   unit  or  standard  must  be 
selected  in  terms  of  which  quantity  maybe  expressed; 

for  example,  the  foot,  the 
pound,  the  second,  the 
horse-power,  all  are  units. 
There  are  two  systems  of 
units:  the  Absolute  and  the 
Practical.  In  the  absolute 
system  all  quantities  are  ex- 
pressed in  terms  of  the  units 
F  of  Mass,  Length,  and  Time. 

Thus,  the   unit  of  length  is 

the  foot,  and  the  distance  from  the  earth  to  the  moon 
is  1,267,200,000  feet,  and  the  velocity  of  a  moving  train 
may  be  88  feet  per  second  expressed  in  the  absolute 
system;  but  it  is  much  more  convenient  to  say  that  the 
moon  is  240,000  miles  away  and  that  the  train  moves  60 
miles  an  hour!  There  thus  arises  a  set  of  units  of 
convenient  multiples  of  the  corresponding  absolute 
units,  constituting  the  system  of  Practical  Units.  The 
practical  units  with  which  we  shall  have  to  do,  are 
those  of  magnetism  and  electricity,  but  before  taking 
them  up  alone,  we  will  see  in  what  way  they  depend 
upon  the  absolute  units  of  Length,  Mass,  and  Time.  The 
unit  of  length,  the  centimetre,  is  about  2/5  of  an  inch; 
that  of  mass,  the  gram,  about  2/1000  of  a  pound;  and 
that  of  time,  the  second;  and  together  are  known  as  the 
C.  G.  S.  system  of  measurement.  The  unit  of  Force  is 
the  Dyne,  and  it  has  been  agreed  to  define  it  as  that  force 


ELEMENTS    OF    THE    DYNAMO.  7 

which,  acting  upon  unit  mass,  for  unit  time,  shall  increase 
its  velocity  by  i  centimetre  per  second,  /.  <•.,  if  upon  a  mass 
of  i  gram  at  rest,  a  force  of  i  dyne  be  allowed  to  act 
for  i  second,  at  the  end  of  the  second  the  gram  weight 
would  be  moving  at  the  rate  of  i  centimetre  per  sec- 
ond.; at  the  end  of  the  next  second  the  velocity  would 
be  i  centimetre  per  second  greater,  and  so  on  for  each 
succeeding  second,  so  the  Acceleration  is  said  to  be  i 
centimetre  per  second.  At  Paris  the  acceleration  of 
gravity  is  981  centimetres  per  second;  the  force  of  grav- 
ity is  therefore  981  dynes,  and  i  dyne  is  1/981  of  the 
force  of  gravity  at  Paris. 

The  more  important  magnetic  units  are: 

i.  Unit  of  Pole  Strength.  A  pole  of  unit  strength  is 
one  which  repels  with  unit  force  an  equal  pole  placed  at 
unit  distance.  Since  at  unit  distance  a  unit  pole  repels 
a  unit  pole  with  a  force  of  i  dyne,  a  pole  of  strength  M 
would  repel  unit  pole  with  a  force  of  M  dynes,  for  each 
unit  of  J/  exerts  upon  the  unit  pole  a  force  of  i  dyne, 
and  there  are  M  units  in  J/.  Likewise  a  pole  of  strength 
M  would  repel  a  pole  of  strength  J/',  J/'  times  as  strongly 
as  it  would  unit  pole,  so  we  can  say  that  at  unit  distance 
two  poles  J/,  J/'  repel  each  other  with  a  force  of  M  J/' 
dynes,  or/  =  M  x  M .  Now  if  the  distance  between  the 
two  poles  is  divided  by  2,  the  force /becomes  four  times 
as  strong;  if  by  3,  nine  times  as  strong,  and  so  on;  while  if 
the  distance  is  doubled  /  becomes  but  1/4  as  great;  if 
trebled,  1/9  as  great,  etc.  By  whatever  amount  the  dis- 
tance, </,  is  affected,  the  force,  /,  is  affected  by  an  amount 
proportional  to  the  reciprocal  of  d*\  in  other  words,  the 
rate  at  which  /is  affected  by  moving  apart  the  poles  in 
question,  can  be  measured  by  multiplying  by  itself  the 


8  TESTING    OF    DYNAMOS    AND    MOTORS. 

reciprocal  of  the  rate  at  which  d  is  affected.  Hence  the 
law. —The  force  between  two  poles  varies  directly  as  the 
product  of  the  pole  strengths,  and  inversely  as  the  square 
of  the  distance;  or, 

MM' 

where  the  --  sign  corresponds  to  a  force  of  attraction 
and  the  -f-  sign  to  one  of  repulsion. 

2.  Magnetic  Field.  If  a  free  pole  could  be  placed  in  a 
magnetic  field,  the  pole  would  move  in  a  definite  direc- 
tion and  with  a  force  depending  upon  the  strength  of  the 
field.  The  force  with  which  a  unit  pole  is  urged  along  is 
a  measure  of  the  field  strength,  and  that  is  unit  field  in 
which  unit  pole  is  acted  upon  with  unit  force.  It  is  to 
Faraday,  who  has  been  well  called 
the  "  Prince  of  Experimenters,"  that 
we  owe  the  development  of  this  sub- 
ject. First,  in  order  to  form  a  mental 
picture  of  a  magnetic  field,  he  consid- 
ered it  to  be  filled  with  imaginary  lines, 
which  everywhere  coincided  in  direc- 
FIG.  2.  tion  with  the  path  of  the  free  pole; 

these  he  called  lines  of  force.  He 
next  mapped  out  a  uniform  field  of  unit  strength,  by 
considering  that  the  lines  were  everywhere  evenly  dis- 
tributed, one  to  each  unit  of  cross-section,  in  a  plane 
normal  to  the  lines.  Thus  each  line  in  any  field  repre- 
sents unit  force,  and  the  number  of  lines  per  unit  of 
cross-section  represents  the  strength  of  the  field.  Fara- 
day next  assigned  to  the  lines  certain  properties  which 
enabled  him  to  explain  many  magnetic  and  electro- 
magnetic phenomena,  (i)  He  considered  that  lines  of 


ELEMENTS    OF    THE    DYNAMO.  ') 

force  tend  to  shorten  in  length,  hence  the  attraction 
between  magnets.  (2)  Lines  going  in  the  same  direction 
repel,  in  opposite  directions  attract  each  other,  hence 
N  poles  repel  N  poles,  but  attract  C  poles.  (3)  If  a 
conductor  be  made  to  cut  lines,  they  resist  its  motion 
just  as  rubber  bands  would.  The  result  of  forcing  a  con- 
ductor across  lines  is  to  produce  in  it  an  E.  M.  F.,  and 
this  phenomenon  is  called  Induction.  (4)  If  a  conductor 
carrying  a  current  be  placed  in  a  magnetic  field,  the  field 
acts  upon  that  due  to  the  current,  and  the  conductor 
moves  in  a  direction  depending  upon  the  direction  of  the 
current  and  the  lines  of  the  field;  by  this  motion,  how- 
ever, the  conductor  is  made  to  cut  lines  of  force,  and, 
therefore,  becomes  the  seat 

of  an  induced  E.  M.  F.      By        -^==j=. -=--^^^^^='-^^^^- 
the  law  of  the  conservation 
of  energy,  given  above,  the 


induced  E.  M.  F.  is  opposed  FIG.  3. 

to     that    which     urges   the 

existing    current,    and    constitutes    the    all    important 

C.  E.   M.   F.   (counter    electromotive    force)    so    much 

heard   of   in   connection    with   motors. 

A  magnetic  pole  produces  about  it  a  magnetic  field, 
aYid  since  unit  pole  exerts  unit  force  at  unit  distance,  the 
field  at  unit  distance  must  be  of  unit  intensity;  /.  ^., 
there  must  be  one  line  of  force  in  each  square  centimetre 
of  sectional  area.  There  will,  therefore,  from  unit  pole 
emanate  as  many  lines  as  there  are  square  centimetres  in 
the  surface  of  the  circumscribing  sphere  of  unit  radius 
(see  Fig.  2).  Now  the  number  of  square  centimetres  in 
the  surface  of  any  sphere  is 

4  X  --  X  r>, 


IO  TESTING    OF    DYNAMOS    AND    MOTORS. 

where  C,  d,  and  r  are  respectively  the  circumference, 
diameter,  and  radius  of  the  sphere,  all  expressed 
in  centimetres.  The  quotient  of  any  circle's  circum- 
ference by  its  diameter  is  the  number  3.1416,  and  is 
designated  by  the  Greek  character,  TT,  called  pi.  The 
area  over  any  sphere  is,  then,  4  n  r*  and  where  r  —  i,  as 
in  the  present  case,  it  is  simply  4  n  X  i  —  4  TT,  which 
expresses  the  number  of  lines  due  to  unit  pole.  The 
number  of  lines  due  to  a  pole  of  strength  M  is  4  n  M. 

3.  Permeability.  If  in  a  magnetic  field  there  be  placed 
a  bar  of  iron,  steel,  or  other  magnetic  metal,  at  the  ends 
of  the  bar  the  field  is  much  stronger  than  elsewhere  (see 
Fig.  3),  showing  that  the  lines  of  force  converge  and 
flow  through  the  metal  in  preference  to  the  air.  This 
property  of  magnetic  "conductivity"  is  of  great  impor- 
tance, and  is  possessed  by  iron  and  its  modifications,  by 
nickel  and  cobalt.  Iron  and  steel,  however,  so  far  sur- 
pass nickel  and  cobalt  in  this  respect  as  to  be  the  only 
magnetic  metals  used  in  electromagnetic  machinery. 
Magnetic  conductivity,  to  distinguish  it  from  electric 
conductivity,  is  called  Permeability,  and  it  may  present 
itself  more  clearly  as  follows:  If  a  current  be  passed 
through  a  coil  of  wire  the  coil  becomes  an  electromagnet, 
and  through  the  space  inside  pass  a  certain  number  of 
lines  of  force,  which  we  will  call  3C.  If  the  internal  air 
space  be  now  filled  by  an  iron  core,  the  coil's  strength  as 
a  magnet  is  greatly  increased;  not  that  the  iron  has 
created  any  lines  of  force,  but  it  has  coaxed  many  of  the 
lines  which  otherwise  would  form  little  circles  about  the 
separate  wires,  to  be  gathered  into  it  and  pass  through 
the  entire  length  of  the  coil  to  manifest  themselves 
at  its  ends. 


ELEMENTS    OF    THE    DYNAMO.  II 

As  far,  then,  as  concerns  the  number  of  lines  passing 
through  a  given  space,  iron  has  in  effect  a  multiplying 
power,  and  this  power,  measured  by  dividing  the  num- 
ber of  lines  which  pass  through  the  iron,  by  the  number 
of  lines  which  pass  through  the  air  space  when  the 
iron,  is  removed  (the  current  and  the  number  of  turns 
used  being  the  same  in  both  cases),  is  called  the  Pcrinc- 
ability,  and  is  designated  by  the  Greek  letter  //,  mu. 

If  JC  is  the  number  of  lines  which  the  given  ampere- 
turns  will  send  through  air,  and  (B  is  the  number  after  the 
iron  is  inserted,  then  the  permeability, 

(B 

»--    jc 

and  this  is  the  formula  usually  given  in  books.  ($>  and  JC 
are  respectively  the  number  of  lines  per  square  centi- 
metre in  iron  and  in  air;  but  where  the  same  cross- 
section  is  considered  in  both  cases-,  as  above,  the  formula 
is  still  true  if  we  regard  (B  and  JC  as  the  total  lines  in 
the  given  section  under  the  two  conditions. 

The  permeability  of  air  remains  always  the  same,  /.  <r., 
the  number  of  lines  of  force  through  air  always  increases 
in  the  same  measure  as  the  magnetizing  force  producing 
them. 

The  following  table  gives  the  relation  between  JC,  (B 
and  p  in  iron.  As  JC  increases  (B  does  not  increase  with 
the  same  rapidity,  and  hence  /<  decreases. 

When  (B  has  reached  such  a  value  that  any  increase  in 
JC  produces  comparatively  little  increase  in  (B,  the  iron  is 
said  to  be  saturated.  For  wrought  iron  this  is  about 
JC  =  105,  (B  =  17,000.  In  cast  iron  the  saturation  point 
is  much  lower,  JC  =  188,  (B  =  10,000.  Above  theso 
values  it  is  unprofitable  to  force  the  magnetization. 


12  TESTING    OF    DYNAMOS    AND    MOTORS. 

TABLE  No.  i  (SQUARE  CENTIMETRE  UNITS).* 


ANNEALED   WROUGHT   IRON. 

CAST   IRON. 

3C 

(B 

n 

3C 

(B 

fj. 

1.66 

5,000 

3,000 

5 

4,000 

800 

4 

9,000 

2,250 

IO 

5,000 

500 

5 

10,000 

2,000 

21.5 

6,000 

279 

6.5 

11,000 

1.692 

42 

7,000 

133 

8-5 

12,000 

1,412 

80 

8,000 

IOO 

12 

13,000 

1,083 

127 

9,000 

71 

17 

14,000 

823 

1  88 

10,000 

53 

28.5 

15,000 

526 

292 

11,000 

37 

50 

16,000 

320 

105 

17,000 

161 

2OO 

18,000 

90 

350 

19,000 

54 

666 

20,000 

30 

It  can  be  seen  why  wrought  iron  is  to  be  preferred  to 
cast  iron  for  magnetic  purposes,  and  particularly  when 
the  cross-section  of  the  field  cores  is  to  be  a  minimum. 
The  best  grades  of  cast  steel  compare  very  favorably 
with  the  best  wrought  iron,  in  regard  to  permeability. 
Some  special  grades  of  steel  have  the  strength  and 
permeability  of  wrought  iron,  and,  being  handled  as 
easily  as  cast  iron,  can  be  used  where  a  forging  would  be 
out  of  the  question.  Steel  castings  are  being  increas- 
ingly used  in  dynamo  and  motor  construction,  and  are 

*  See  S.  P.  Thompson's  Electromagnet,  p.  67. 

NOTE. — The  statement  that  3C  in  the  iron  is  the  field  strength  in  the 
air  is  not  rigidly  true.  Whenever  a  piece  of  iron  or  steel  is  placed  in  a 
magnetic  field,  the  metal  itself  becomes  a  magnet  of  such  polarity  as  to 
oppose  the  existing  field  3C-  Hence  the  actual  strength  of  JC  in  tne 
iron  is  slightly  less  than  in  air. 


ELEMENTS  OF  THE  DYNAMO.  13 

especially  well  adapted  to  frames  of  intricate  shape. 
With  some  manufacturers  cast-iron  frames  are  cast 
over  laminated  wrought-iron  pole-heads;  with  others 
wrought-iron  pole-heads  are  bolted  onto  the  cast-iron 
frame. 

The  terms  ampere,  volt,  and  ohm  are  familiar  to 
most  of  us,  as  they  occur  in  Ohm's  law.  If  a  wire  be 
connected  to  the  terminals  of  a  cell  a  current  flows. 
This  flow  is  not  instantaneous,  but  involves  time,  and  this 
fact  is  implicitly  stated  when  we  say  a  wire  has  resist- 
ance. By  increasing  the  length  of  a  wire,  we  increase 
its  resistance;  l;y  increasing  the  cross-section  of  the 
wire,  its  resistance  is  decreased  in  a  like  measure: 
whence  we  can  say  that  the  resistance  of  any  wire  varies 
directly  as  its  length  and  inversely  as  its  cross-section, 
or  since  its  cross-section  is  .7854  •</',  or  }£  •  n  -  </*, 
inversely  as  the  square  of  its  diameter  d;  i.  e.,  if  the 
length  is  doubled,  so  is  the  resistance,  but  if  the  diameter 
is  doubled,  the  cross-section  is  quadrupled,  and  the 
resistance  is  but  one  quarter  as  great.  With  all  solid 
conductors,  except  carbon,  the  resistance  rises  when  the 
temperature  does,  but  of  carbon  and  liquids,  the  reverse 
is  true.  Striking  examples  of  these  exceptions  are  met 
with  in  using  incandescent  lamps  and  water  boxes  for 
loading  dynamos  under  test.  On  either  of  these  devices 
the  current  will  creep  above  its  initial  value,  but  the 
effect  is  most  marked  on  the  water  box. 

The  practical  unit  of  resistance  is  the  Ohm  and  its 
symbol  (R  or  r.  As  defined  by  the  Electrical  Congress 
held  in  Chicago  in  1893,  it  is  as  follows:  "The  unit  of 
resistance  shall  be  known  as  the  International  Ohm,  and  is 
reoresented  by  the  resistance  offered  to  an  unvarying 


14  TESTING    OF    DYNAMOS    AND    MOTORS. 

electric  current  by  a  column  of  mercury,  at  the  tempera- 
ture of  melting  ice,  14.4521  grams  in  mass,  of  constant 
cross-section  and  of  a  length  of  106.3  centimetres." 
The  absolute  unit  and  its  relation  to  the  practical  unit 
will  be  given  later. 

The  quantity  of  electricity  which  passes  through  a 
given  conductor  depends  upon  the  rate  and  duration 
of  flow.  The  quantity  per  second  is  the  rate  of  flow 
and  indicates  the  Current  Strength.  The  international 
unit  is  based  upton  the  experimental  fact  that  a  given 
quantity  of  electricity  passing  through  a  solution  of  a  sil- 
ver salt,  decomposes  the  salt  and  deposits  a  fixed  amount 
of  metallic  silver  upon  the  negative  plate.  The  unit  of 
current  is  the  Ampere,  and  is  that  current  which  deposits 
silver  at  the  rate  of  .0001118  of  a  gram  per  second.  Its 
symbol  is  /  or  /,  but  C  is  very  commonly  used. 

Since  all  conductors  have  resistance,  force  is  required  to 
urge  a  current  through  them.  Whatever  causes  or  tends 
to  cause  a  flow  of  electric  current  is  called  an  Electromotive 
Force.  The  term  electrical  pressure  is  also  used.  On 
a  conductor  through  which  a  current  flows,  successive 
points  are  said  to  vary  in  potential,  and  according  to  a 
well-defined  law.  The  potential  falls  in  the  direction  of 
flow,  and  is  a  minimum  at  the  negative  pole  of  the  source. 
Two  points  may  be  said  -to  be  at  different  potentials,  and 
the  degree  of  difference  is  called  the  "  drop  "  of  poten- 
tial between  the  points.  The  practical  unit  of  poten- 
tial difference  corresponding  to  the  ohm  and  ampere  is 
the  international  Volt,  and  is  officially  defined  as  the 
electromotive  force  that,  steadily  applied  to  a  conductor 
whose  resistance  is  i  ohm,  will  produce  a  current  of 
i  ampere.  Its  symbol  is  E  or  e. 


ELEMENTS  OF  THE  DYNAMO.  15 

Ohm's  law,  to  be  analyzed  later,  gives  the  relation 
which  exists  between  /,  E,  and  R,  in  any  part  of  a  cir- 
cuit, and  in  words  states  that  the  rate  of  transfer  of 
electricity  in  any  part  of  a  circuit  is  directly  proportional 
to  the  difference  of  potential  between  the  points  taken, 
and  is  inversely  proportional  to  the  resistance  between 
these  points.  Since  E  =  I  A\  the  "drop"  or  loss  of 
potential  along  a  line  can  be  found  if  /and  /?are  known. 

The  distinction  between  current,  /,  and  quantity  of  elec- 
tricity (denoted  by  0,  should  be  clearly  grasped.  The 
clerk  who  said  that  his  company  would  furnish  lighting 
at  one  cent  per  ampere  was  confusing  current  with  quan- 
tity, and  was  as  much  in  error  as  the  small  boy  would  be, 
should  he,  in  answer  to  the  question  as  to  how/V/r  he  had 
walked,  reply,*'  Four  miles  an  hour."  The  unit  of  current, 
the  ampere,  we  have  defined;  the  unit  of  quantity  is  the 
Coulomb;  and  is  the  quantity  of  electricity  which  passes  any 
given  cross-section  of  a  conductor,  when  a  current  of  i 
ampere  flows  for  i  second,  1/2  ampere  for  2  seconds, 
or  2  amperes  flow  for  1/2  second,  and  so  on.  The  prac- 
tical unit  of  quantity  is  the  Ampere-Hour,  and  since  the 
hour  contains  3,600  seconds,  i  ampere-hour  is  equiva- 
lent to  3,600  coulombs. 

Electrical  power,  or  rate  of  doing  work,  depends  upon 
7  and  £,  and  is  equal  to  their  product.  This  may  be 
illustrated  by  the  analogy  of  water  pressure.  The  head 
of  water  corresponds  to  /,  and  the  pressure  in  the  pipe 
to  E,  and  just  as  the  power  of  the  water  in  the  pipe 
equals  the  head  of  water  x  pressure,  so  electrical  power 
=  E  X  /.  Since  E  —  I R  the  expression  for  watts  be- 
comes watts  =  /  X  I R  =  /a  R.  The  unit  of  electrical 
power  is  the  Watt  (or  Kilowatt  =.  1,000  watts),  and  its 


l6  TESTING    OF    DYNAMOS    AND    MOTORS. 

symbol,  P,  although  IV  is  frequently  used.  Energy  is 
capacity  for  doing  work,  and  is  measured  in  the  same 
units.  The  amount  of  electrical  energy  consumed,  or 
work  done,  depends  upon  time  and  power;  i.  e.,  upcn  the 
number  of  seconds  and  the  work  done  in  one  second. 
The  unit  of  power,  or  working  rate,  we  have  seen  to 
be  the  watt;  the  unit  of  time,  the  second;  so  the  unit 
of  work  must  be  (rate  X  time)  watts  x  seconds,  or,  as 
we  may  say,  Watt- Seconds.  The  unit  most  used  is  larger 
than  this,  and  is  the  Watt-Hour.  The  watt  second  is 
called  the  Joule,  and  i  watt-hour  =  3,600  joules.  The 
same  distinction  that  has  been  made  between  the  ampere 
and  the  coulomb  must  be  observed  between  the  joule 
and  the  watt.  The  watt  is  the  unit  of  power,  or  is  the 
work  done  per  unit  of  time,  while  the  joule  is  a  given 
amount  of  work  irrespective  of  the  time  it  may  take 
to  do  it. 

Joules  —  Work  done  =  Power  x  Time  =  Watts  X  Sec- 
onds. The  mechanical  unit  of  work  is  the  work  done  in 
lifting  i  pound  through  i  foot  against  gravity,  and  when 
work  is  done  at  such  a  rate  that  550  pounds  are  lifted 
through  i  foot  in  i  second  the  rate  is  i  Horse  Power, 
which  is  the  mechanical  term  analogous  to  watt.  Now, 
the  amount  of  work  done  is  the  same  whether  550 
pounds  are  lifted  i  foot,  or  1,100  pounds  1/2  foot,  and 
it  does  not  matter  whether  the  work  is  done  in  i  second 
or  i  hour;  but  to  do  work  at  the  rate  of  i  HP,  the 
equivalent  of  550  pounds  must  be  lifted  through  i  foot  in 
i  second  of  time.  The  difference  between  work  and 
power,  watt-hours  and  watts,  cannot  be  too  carefully 
noted,  as  any  confusion  is  a  fruitful  source  of  error. 

The  values  of  the  practical   units  of  E,  7,  and  R  have 


ELEMENTS    OF    THE    DYNAMO.  17 

been  given.  In  establishing  the  units  of  the  absolute 
system,  upon  which  the  practical  system  is  based,  use  is 
made  of  the  relation  existing  between  the  electric  current 
and  magnetism.  As  early  as  1819  Oersted  observed  that 
a  compass  needle  in  the  neighborhood  of  a  wire  carrying 
current  was  influenced  by  it,  tending  to  take  up  a  posi- 
tion at  right  angles  to  the  wire.  He  observed  also  that 
this  influence  was  increased  by  using  a  coil  of  many  turns; 
if  a  current  be  made  to  flow  through  ten  turns,  its  effect 
upon  the  needle  placed  at  the  centre  of  the  coil  is  ten 
times  that  of  one  turn.  Upon  this  fact  is  based  the  defi- 
nition of  current,  as  follows:  Con- 
sider a  unit  pole  at  the  centre  of  a 
coil  of  a  single  turn  whose  radius 
is  unity  (see  Fig.  4).  The  length 
of  the  coil  is  3.1416  x  diameter 
—  n  d  =  2  n  r,  expressed  in  centi- 
metres. Since  we  assume  that /•=  i, 

i*  IG, 

the  length  of  wire  is  2  TC  centimetres. 
If  there  now  be  sent  around  the  coil  a  current  of  such 
strength  as  to  exert  upon  unit  pole  at  the  centre  a  force 
of  i  dyne  for  each  centimetre  of  wire  in  the  coil,  the 
current  is  said  to  have  unit  value.  There  are  2  n  units  of 
length  in  the  wire,  so  that  the  force  exerted  by  the  whole 
turn  is  2  n  dynes.  This  current  is  the  absolute  unit: 
the  practical  unit  or  International  Atnpfre  is  i/io  of  this 
current. 

Before  Oersted's  time  magnetic  fields  were  produced  by 
means  of  natural  magnets  only,  or  steel  magnets  charged 
from  them.  His  experiment  led  to  the  discovery  that 
a  piece  of  iron  or  steel  could  be  magnetized  by  means  of 
an  electric  current,  and  to  a  much  higher  degree  than  by 


i8 


TESTING    OF    DYNAMOS    AND    MOTORS. 


the  old  method.  The  piece  to  be  magnetized  is  placed 
within  a  coil  of  wire,  and  a  heavy  current  sent  through 
the  coil.  By  means  of  Faraday's  conception  01  lines  of 
force  the  effect  is  readily  understood.  He  supposed  that 
surrounding  every  electric  current  were  lines  of  force 
disposed  in  concentric  circles,  as  in  Fig.  5.  With  the 
current  passing  down  through  the  plane 
of  the  paper,  the  lines  pass  around  the 
wire  clockwise,  as  indicated  by  a  com- 
pass needle.  If  instead  of  one  there 
be  many  turns,  the  total  number  of 
lines  offeree  is  proportionately  greater. 
The  effect  of  introducing  an  iron  core 
into  the  coil,  we  have  seen  above.  At 
either  end  of  such  an  electromagnet  the  field  is  very 
strong  (see  Fig  6).  Fig.  7  gives  a  still  better  form:  here 
the  magnetic  circuit  is  all  of  iron,  except  the  small  air 
space,  N  S.  The  relation  between  the  magnetizing 
force  =  current  X  turns  =  ampere  turns,  (usually 
designated  as  S  i  )  and  the  resulting  number  of  lines, 
3C,  due  to  the  coil  in  air,  is  as  follows: 


FIG.  5. 


ae  = 


10 


x      = 

L 


x     • 

L 


where  3C  is  the  number  of  lines  of  force  per  square  centi- 
metre, or  strength  of  field  due  to  the  ampere-turns  Si,  ex- 
clusive of  the  influence  of  the  iron,  and  L  is  the  average 
length  of  the  magnetic  circuit.  The  cross-section  of  the 
circuit  does  not  enter  this  expression  because  the  path  is 
air  (before  the  ring  is  inserted),  and  since  the  permeability 
of  air  is  unity,  increasing  the  cross-section  does  not  in- 
crease the  capacity  for  lines  of  force,  but  simply  decreases 


ELEMENTS    OF    THE    DYNAMO. 


their  density.     As  soon  as  iron  is  introduced  into  the  coil 
the  multiplying  power  of  the  iron  becomes  an  important 
factor  and  interests    us    in 
(B,  the  field  strength  in  the 
iron,  rather  than  in  JC,  that 
in  air.     To  find  (B,  we  must   ^: 
know  JC,  or  find  it  from  the  ^~ 
above  formula.      For  exam- 


ple, if  5  and  7  centimetres  ^^ 

are  respectively  the  inside  FIG."^" 

and    outside    diameters   of 

the  ring,  the  average  diameter  will  be  6  centimetres  and 
the  average  path,  Z,  for  lines  of  force  6  n  centimetres, 
or  18.8  centimetres  =  Z.  If  the  current  (/')  =  5  am- 
peres, and  the  number  of  turns  (S)  =  50,  the  expression 
for  3C  becomes, 

_     1.26  X  50  X  5  „  l6  7 
18.8 

Now  in  many  cases  to  get  (B  from  3C  we  would  have 
to  know  the  permeability  (//)  of  the  metal  in  question, 
but  since  all  samples  of  good  wrought 
iron  are  very  similar  in  magnetic  prop- 
erties, we  can  assume  that  Table  I.  gives 
close  enough  results,  and  from  it  derive 
the  value  of  (B  corresponding  to  any 
given  value  of  JC. 

Thus  for  3C  =  16.7,  we  see  (B  =  13,900. 
Where  the  magnetic  circuit  is  not  all 
FJG   7  iron,  but  is  part  air,  part  cast  iron  and 

part  wrought  iron,  as  in  some  actual 
machines,  the  3C  necessary  to  force  the  required  number 
of  lines  through  each  part  is  determined,  and  the  sum  of 


20  TESTING    OF    DYNAMOS    AND    MOTORS. 

these  3C's  is  the  Magnetomotive  Force  which  must  be 
provided  to  get  the  desired  number  of  lines  through  the 
magnetic  circuit.  The  magnetic  circuit  cannot  be  treated 
as  a  whole  because  the  permeability  of  its  parts  is  not  the 
same.  The  product  6"  /  designates,  as  we  know,  Ampere- 
Turns,  and  to  secure  a  given  magnetic  effect  it  makes  no 
difference,  except  in  cost  of  manufacture,  whether  £  i  be 
composed  of  many  turns  and  small  current,  or  of  few 
turns  and  large  current,  provided  that  in  any  case  the 
product  6*  i  is  not  changed.  One  ampere  flowing  around 
an  iron  core  ten  times  has  the  same  magnetic  effect  as  ten 
amperes  flowing  around  once.  In  actual  designing  there 
are  considerations  which  dictate  what  arrangement  of 
current  and  turns  shall  be  used  to  obtain  a  desired  mag- 
netization. 

If  a  piece  of  iron  or  steel  be  placed  in  a  coil  through 
which  current  is  made  to  gradually  vary  from  zero 
to  a  maximum,  and  back  to  zero,  it  is  found  that  upon 
removing  the  current  the  magnetization  does  not  fall  to 
zero,  but  remains  to  a  marked  degree.  This  property 
which  iron  and  steel  have  of  retaining  magnetism  is 
called  retcntivity)  and  the  magnetism  retained  is  known 
as  residual  magnetism.  Soft  iron  is  more  readily  mag- 
netized than  steel,  but  parts  with  its  magnetism  more 
readily  also.  Or  we  may  say  it  is  more  permeable  than 
cast  iron  or  hard  steel,  but  is  not  so  retentive.  It  is  by 
virtue  of  their  residual  magnetism  that  dynamo  fields  (to 
not  ordinarily  require  an  initial  charge  each  time  they  are 
used. 

Following  on  the  track  of  Oersted,  Ampere,  in  1821, 
advanced  the  following  principles,  which  he  proved  ex- 
perimentally: i.  Parallel  currents,  if  flowing  in  the  same 


ELEMENTS    OF    THE    DYNAMO.  21 

direction,  attract,  and  if  flowing  in  opposite  directions, 
repel,  each  other.  The  explanation  lies  in  the  reaction  of 
the  magnetic  fields  surrounding  each  current  (see  Figs.  8 
and  9),  and  it  follows  from  Faraday's  conception  of  lines 
of  force  oppositely  directed  attracting  each  other,  that 
lines  of  force  cause  an  attraction  between  conductors. 
It  must  be  borne  in  mind,  however,  that  the  currents 
exert  the  attractive  force  and  not  the  conductors;  the 
latter  are  simply  dragged  along  with  the  currents. 

In  1831  Faraday  performed  an  experiment  which  sealed 
the  doom  of  all  chemical  sources  of  electricity  as  applied 
to  light  and  power.  He  discovered  Electromagnetic  Indue- 
tion.  If  a  conductor  forming  part  of  a  closed  gal- 
vanometer circuit  be  made  to  cut  the  lines  of  force  of  a 
magnetic  field,  the  conductor  becomes  the  seat  of  a  cur- 
rent as  indicated  by  the  swing  of  the  galvanometer  needle. 
The  current  is  generated  only  while  the  conductor  is  in 
motion,  and  its  direction  depends  upon  that  of  the  motion. 
It  is  evident  that  the  cause  of  the  current  is  the  motion 
of  the  conductor,  and  the  sole  condition  necessary  is  that 
the  conductor  cut  lines  of  force.  Even  if  the  circuit  be 
open  there  is  still  present  the  force  tending  to  produce  a 
current.  This  force,  then,  fulfills  the  definition  of  an 
electromotive  force  in  that  it  causes  or  tends  to  cause  a 
flow  of  electric  current.  The  following  statement  is  then 
true :  There  is  an  E.  M.  F.  present  in  every  conductor  cut- 
ting lines  of  force.  Furthermore,  the  value  of  this 
E.  M.  F.  will  depend  upon  the  number  of  lines  cut  per 
unit  of  time,  and  this,  In  turn,  upon  the  length  of  the 
conductor,  the  field  strength,  and  the  rate  of  motion. 

Let  the  circuit  be  closed  through  the  unit  resistance 
already  given,  and  let  the  conductor  velocity  be  raised 


22  TESTING    OF    DYNAMOS    AND    MOTORS. 

until  a  current  of  one  ampere  flows,  the  E.  M.  F.  will 
now  be  one  volt.  The  volt  may  now  be  defined  in  terms 
of  the  number  of  lines  of  force  cut  per  second,  and  is  in 
fact  the  E.  M.  F.  generated  by  a  conductor  which  cuts 
100,000,000  lines  per  second.  Observe  that  the  length 
of  the  conductor  (its  resistance  being  supposed  zero), 
does  not  enter,  for  according  as  it  is  long  or  short  the 
velocity  is  correspondingly  less  or  greater,  the  area 
swept  through  and  the  number  of  lines  cut  remaining  the 
same.  The  absolute  or  C.  G.  S.  unit  of  E.  M.  F.  is  that 
produced  in  a  conductor  cutting  one  line  of  force  per 
second.  Having  now  the  absolute  units  of  current  and 
E.  M.  F.,  Ohm's  law  gives  the  corresponding  absolute 
unit  of  resistance, 


and  is  that  resistance  through  which  unit  E.  M.  F.  will 
urge  unit  current. 

Moving  a  conductor  in  a  magnetic  field  is  one  way  of 
obtaining  induction.  In  this  method,  the  conductor  is 
the  active  agent  and  cuts  the  lines  of  force.  A  second 
method  provides  a  moving  field,  and  here  the  lines  of 
force  are  active  and  cut  the  conductor.  If  two  coils  be 
placed  side  by  side,  and  a  variable  or  alternating  current 
be  sent  through  one  of  them,  there  will  be  produced  a 
variable  field  growing  stronger  and  extending  further  out 
as  the  current  strengthens,  and  closing  in  on  itself  as 
the  current  weakens.  This  variation  in  field  strength 
causes  lines  of  force  to  move  and  cut  the  conductors  of 
the  second  coil,  with  a  resulting  current  therein  if  its 
circuit  be  closed. 


ELEMENTS    OF    THE    DYNAMO. 


FIG.  8. 


the 


The  cutting  of  one  conductor  by  lines  of  force  due  to 
current  in  another  conductor  constitutes  what  is  called 
Mutual  Induction,  and  forms 
the  underlying  principle  up- 
on which  is  based  all  trans- 
former construction. 

The  lines-of  force  also  cut 
the  first  coil,  tending  to  pro- 
duce in  it  a  current  opposed 
to  that  already  flowing;  this 

effect   is   known  as  self-induction^    and    to  it  is  due 
spark    seen  upon   breaking  a  field  circuit. 

Passing  from  the  abstract  theory  to  its  more  practical 
application,  the  experiment  of  the  disc  can  now  be  readily 
explained. 

Let  us  consider  the  disc  as  made  up  of  conductors 
radiating  from  the  axle  outward.  It  is  seen  at  once  that 
we  have  here  conductors  cutting  lines  of  force,  and  as 
the  conductors  pass  under  the  brush  (Fig.  i)  the  circuit 
is  closed  and  a  flow  of  current  results. 
The  maximum  current  would  be  obtained  by  using  a 

brush  long  enough 
to  include  all  the 
conductors  in  the 
field,  or  what  would 
be  the  same  in  ef- 
fect, have  several 
brushes  arranged 
FIG.  9.  around  the  periph- 

ery of  the  disc  and 

connect  them  together.     It  may  be  noted  that  while  this 
explains  the  action  of  the  wheel,  it  does  not  answer  the 


24 


TESTING    OF    DYNAMOS    AND    MOTORS. 


Y' 


FIG. 


fundamental  question  as  to  why  an  E.  M.  F.  is  produced 
when  a  conductor  moves  in  a  magnetic  field.  This  ques- 
tion has  never  been  answered. 

We  have  now  all  the  elements  of 
a  dynamo,  and  need  only  to  assemble 
them.  In  the  first  place  there  must 
be  a  magnetic  field.  This  the  field 
coils  and  frame  provide,  for  the  fields 
are  simply  powerful  electromagnets 
whose  lines  of  force  flow  around  the 
frame  and  across  the  path  of  the  ar- 
mature wires.  To  secure  high  per- 
meability the  cores  C,  C',  Fig.  10,  are 
made  of  wrought  iron  or  soft  steel; 
the  yoke  Y  Y'  is  preferably  of  the 
same  material.  P  and  P't  if  of  cast  iron,  are  of  relatively 
larger  cross-section,  for  since  cast  iron  is  less  permeable, 
there  must  be  more  square  centimetres  for  the  given 
number  of  lines  to  pass  through  than  in  the  wrought  iron 
cores.  In  passing  from  a  medium  of  high  to  one  of  lower 
permeability,  there  is  a  tendency  to  thronle  the  lines  of 
force  at  the  joint;  to  minimize  this 
effect,  some  makers  flange  the  good 
conductor,  as  in  Fig.  IT  ;  others  prefer 
to  set  it  in  as  in  Fig.  12.  In  either 
case  the  idea  is  to  increase  the  cross- 
section  of  the  joint.  To  decrease 
magnetic  leakage  and  other  losses  in- 
herent in  iron,  it  is  customary  to  build 
the  armature  core  of  soft  sheet  iron, 
bility  of  the  metal  part  of 


FIG.  ii. 


The    permea- 
the  circuit  depends  upon 
the  degree  of  saturation  to  which  the  metal  is  worked. 


ELEMENTS  OF  THE  DYNAMO.  25 

Under  the  working  conditions  of  present-day  machines, 
the  number  of  lines  per  square  centimetre  cross-section, 
/.  e. ,  the  induction  (B,  is  about  16,000  in  wrought  iron,  corre- 
sponding to  a  permeability  of  about  320.  This  means 
that  the  permeability  of  the  metal 
part  of  the  magnetic  circuit,  when  it 
is  carrying  the  necessary  number  of 
lines,  is  320  times  that  of  air.  By  far 
the  greater  portion  of  the  magnetiz- 
ing force,  JC,  is  employed  in  forcing 

the  lines  across  the  air  gap.     When 

,  .  •  FIG.  12. 

05  =  16,000  in  wrought  iron,  as  above, 

the  same  ampere-turns  would  send  the  same  magnetism 
through  320  inches  of  iron   as  through  i  inch  of  air. 

The  next  requirement  is  a  set  of  conductors  to  cut  the 
lines  of  force  of  the  field.  The  armature  fulfills  this 
requirement.  On  the  surface  of  an  iron  cylinder  are 
placed  conductors  which  cut  the  lines  twice  in  every 
revolution  of  the  cylinder.  Let  a,  b,  cy  d.  Fig.  13,  be  a 
coil  revolving  in  a  magnetic  field;  all  the  wires  in  a,  by 
cut  upward  through  the  field,  and  their  several  E.  M. 
Fs.  are  in  the  same  direction — from  a  to  b.  The  side 
c,  d)  however,  cuts  downward,  and  its  resulting  E.  M. 
Fs.  are  in  the  direction  c  to  </,  opposed  to  those  in  </,  b, 
as  regards  a  point  in  space,  but  concurring  as  regards  the 
total  circuit  a,  b,  c,  d;  i.  c.,  all  the  conductors  of  the  coil 
are  in  series  and  the  total  E.  M.  F.  is  the  sum  of  the 
separate  E.  M.  Fs.  in  each  wire.  When  the  plane  of  the 
coil  is  vertical,  the  motion  of  the  conductors  is  parallel  to 
the  direction  of  the  lines,  and  the  conductors  slide  along 
rather  than  cut  the  lines.  The  E.  M.  F.  is  therefore 
zero  when  the  plane  of  the  coil  is  vertical.  In  the  hori- 


26  TESTING    OF    DYNAMOS    AND    MOTORS. 

zontal  position  the  number  of  lines  cut  is  a  maximum, 
and  the  E.  M.  F.  is  also  a  maximum.  The  E.  M.  F.  is 
reversed  in  each  side  of  the  coil  as  it  crosses  the  vertical 
plane,  for  here  one  side  ceases  to  cut  up  and  commences 
to  cut  down,  and  the  other  side -vice  versa. 

If  the  coil  be  a  closed  circuit  it  will  carry  a  current 
which  is  half  the  time  in  one  direction  and  half  in  the 
other,  constituting  what  is  called  an  alternating  current. 


If  instead  of  closing  the  armature  circuit  by  bringing 
its  -f-  and  —  ends  together,  we  bring  them  out  to  col- 
lector rings  to  which  are  attached  also  the  terminals  of  an 
outside  circuit,  there  will  be  in  this  circuit  an  alternating 
E.  M.  F.,  the  number  of  single  alternations  per  second 
being  the  same  as  the  number  of  poles  multiplied  by 
the  number  of  revolutions  per  second.  A  set  of  such 
coils  properly  disposed  and  connected  to  collector  rings 
on  the  shaft  constitute  the  ordinary  alternating  current 
machine.  If  it  is  desired  to  have  the  line  E.  M.  F. 
direct,  /.  e.,  always  in  the  same  direction,  there  must  be 
used  a  special  device  called  a  commutator.  In  Fig.  13, 
suppose  that  during  the  first  half  revolution  the  line 
E.  M.  F.  and  current  are  in  the  direction  of  the  arrows: 
unless  something  is  done  to  prevent  it  the  E  and  /  will 


ELEMENTS    OF    THE    DYNAMO.  2"J 

reverse  in  the  second  half  of  the  revolution.  If,  how- 
ever, as  the  coil  passes  through  the  zero  position  the 
terminals  of  the  coil  be  reversed  with  respect  to  the 
brushes,  this  will  keep  the  end  of  the  coil  which  corre- 
sponds to  the  right  side  of  the  armature  always  in  contact 
with  brush  i,  and  the  same  side  of  the  line;  and  the  end 
of  the  coil  which  corresponds  to  the  left  side  in  contact 
with  brush  2,  thus  keeping  the  E.  M.  F.  on  the  line 
always  in  the  same  direction.  Effecting  this  change  is, 
as  the  name  implies,  the  duty  of  the  commutator,  and 
the  fundamental  distinction  between  the  direct  and  the 
alternating  current  machine  is  that  the  former  has  a 
commutator,  while  the  latter  has  not. 

On  direct  current  machines,  brushes  are  set  with  due 
regard  to  the  neutral  or  non-sparking  points,  and  occupy 
a  position  midway  between  pole  pieces,  unless  fur 
mechanical  reasons  the  armature  connecting  wires  are 
given  a  lead  so  as  to  bring  the  brushes  elsewhere.  On 
bipolars  these  points  are  diametrically  opposite,  but  on 
multipolars  the  angle  to  be  included  between  brushes 
equals  the  quotient  obtained  by  dividing  360°,  the  cir- 
cumference of  the  armature,  by  the  number  of  poles. 
On  an  alternator,  the  brushes  are  placed  in  any  position 
easy  of  access. 

The  principle  of  the  production  of  an  E.  M.  F.  by  con- 
ductors cutting  lines  by  force  was  known  twenty  years 
before  the  construction  of  a  machine  that  could  be  fairly 
said  to  resemble  the  modern  dynamo.  In  1856  Siemens 
of  Berlin  patented  his  shuttle-wound  drum  armature. 
The  winding  consisted  of  a  single  coil  connected  to  a  two- 
part  commutator.  Its  action  can  be  followed  by  the  aid 
of  Fig.  14.  We  learned  above  that  when  the  conductors 


28  TESTING    OF    DYNAMOS    AND    MOTORS. 

slide  along  the  lines  of  force  they  give  rise  to  no  E.  M.  F. , 
or  the  E.  M.  F.  is  o,  and  this  value  corresponds  to  posi- 
tion a  in  Fig.  14.  Now,  as  the  conductor  leaves  the 
neutral  field,  it  becomes  the  seat  of  an  E.  M.  F.  which 
increases  and  becomes  greatest  when  the  conductors  cut 
the  lines  at  right  angles,  and  this  condition  is  represented 
by  point  b.  With  each  revolution  the  E.  M.  F.  rises  to 
a  maximum,  falls  to  zero,  rises  to  an  opposite  maximum, 
when  the  coil  cuts  the  lines  upward,  and  again  falls  to  zero. 
In  Fig.  14  let  a  e  represent  the  time  of  one  revolution. 

6 


Then  one-quarter  of  the  length  a  e  will  represent  one- 
quarter  revolution.  Starting  at  a  with  the  coil  at  the 
neutral  line,  the  E.  M.  F.  rises  to  a  positive  maximum  b 
in  one-quarter  revolution,  falls  to  zero  at  <r,  rises  to  a 
negative  maximum  at  d  (i.  e.,  E.  M.  F.  is  reversed),  and 
again  returns  to  zero  after  one  complete  revolution.  Fig. 
15  shows  the  effect  of  using  the  commutator.  The  part 
of  the  curve  c  d  e  has  been  reversed  so  that  both  maxima 
are  of  the  same  polarity,  in  the  outside  circuit,  and  now 
occur  at  b  and  d.  The  internal  armature  changes  remain 
as  before,  but  the  character  of  the  line  E.  M.  F.  has  been 
changed  from  alternating  to  pulsating. 

The  defect  of  the  Siemens  machine  is  that  its  E.  M.  F. 


ELEMENTS  OF  THE  DYNAMO.  29 

is  too  unsteady.  But  if  a  second  coil  be  placed  at  right 
angles  to  the  first,  it  will  have  its  greatest  E.  M.  F.  when 
that  of  the  first  coil  is  zero,  the  two  coils  being  always 
just  a  quarter  revolution  apart.  These  facts  are  shown 
in  the  dotted  line  of  Fig.  15.  The  E.  M.  F.  in  each  coil 
still  fluctuates  between  zero  and  a  maximum,  but  the  line 
E.  M.  F.  does  not  fall  below  ^,  though  the  fluctuation 
is  still  marked.  The  resultant  E.  M.  F.  is  shown  by  the 
curve  Ab  B b'  etc.  Each  additional  coil  narrows  the 
range  of  variation,  so  that  although  each  coil  has  its  rise 


FIG.  15. 

and  fall  of  E.  M.  F.,  the  E.  M.  F.  at  the  brushes  is  prac- 
tically constant. 

E.  M.  F.  is  dependent  upon  speed,  number  of  armature 
conductors,  and  field  strength.  Increase  any  of  these 
and  the  E.  M.  F.  is  increased.  Constancy  of  speed  de- 
pends upon  the  engine  (and  perhaps  belt  or  clutch),  and 
can  be  closely  regulated  from  no  load  to  full  load.  The 
number  of  conductors  on  a  given  armature  is  fixed. 
The  production  and  maintenance  of  the  magnetic  field  is 
the  remaining  factor.  There  are  four  distinct  methods 
of  excitation,  and  a  fifth,  which  is  a  combination  of  two 
others,  i.  Excitation  from  permanent  magnets;  2.  so- 
called  separate  excitation;  3.  series;  4.  shunt;  5.  com- 
pound (C.  IV.)  i.  f.j  part  series,  part  shunt  excitation. 


30  TESTING    OF    DYNAMOS    AND    MOTORS. 

The  first  method  has  its  only  present  practical  appli- 
cation in  the  familiar  magneto  machine,  provided  with  a 
steel  horseshoe  magnet.  The  second  method  consists  in 
connecting  the  field  winding  to  some  external  source  of 
supply — batteries,  or  to  another  machine.  Batteries  are 
out  of  the  question,  and  to  presuppose  another  machine 
is  to  again  raise  the  question  of  excitation.  Separate 
excitation  is  adapted  to  testing-room  practice  on  account 

of  its  steadiness  and  indepen- 

/^\      ^--^-^  dence.     It  also    meets   the  re- 

\^s        UUU(j  quirements     of     special    cases 

outside,     and     is     most     seen 
where   machines  feed  into   the 
same  ^  bars;    here   all   fields 
FIG.    16.  connect    directly   to    the    bars, 

and  each  machine  may  be  re- 
garded as  separately  excited  till  its  own  switch  is  closed. 
The  third  method  is  shown  in  Fig.  16.  Armature, 
field,  and  line  are  in  series,  and  the  total  current  passes 
through  the  exciting  coils.  But  few  turns  are  therefore 
needed  to  produce  the  required  ampere-turns,  and  the 
coils  are  of  heavy  wire  -to  minimize  losses  incidental  to 
all  wires  carrying  current. 

Shunt  machines  form  the  fourth  type.  As  the  name 
implies,  the  field -winding  is  a  shunt  to  something,  and 
in  fact  does  shunt  the  line.  On  isolated  machines  the 
field  connection  is  made  below  the  line  switch,  so  that 
upon  opening  the  latter  the  field  is  not  broken.  On 
starting  up,  a  slight  E.  M.  F.  exists,  due  to  the  residual 
field,  and  a  small  current  therefore  flows  around  the  field 
coils,  increasing  their  magnetization;  this  in  turn  raises 
the  E.  M.  F.,  till  finally  it  reaches  its  normal  value  for  the 

\ 


ELEMENTS    OF    THE    DYNAMO.  31 

given  speed.  The  final  field  current  equals  the  E.  M.  F. 
at  the  terminals,  divided  by  the  resistance  of  the  field 
circuit;  anything  which  lowers  the  E.  M.  F.  lowers  the 
field  ampere-turns,  this  reacting  again  on  the  E.  M.  F. 
We  have  learned  that  current  /,  passing  through  a 
resistance,  A*,  expends  energy  at  the  rate  of  P  R  watts 
per  second,  and  that  the  loss  of  potential  is  I  R.  This 
loss  of  potential  takes  place  in  the  armature  as  well  as  in 
the  rest  of  the  circuit.  As  the  load  increases  the  arma- 
ture loss  increases,  the  terminal  voltage  decreases,  and 
with  it  the  field  current,  whose  weakening  causes  a 
further  decrease  in  E.  M.  F.  At  full  load  the  voltage  is 
then  lowest,  at  no  load  it  is  highest,  while  on  a  series 
machine  the  reverse  is  true. 

The  compound-wound  dynamo  is  the  shunt  and  series 
machine  in  one;  on  open  circuit  it  is  shunt  excited, 
while  on  full  load  the  series  winding  has  its  greatest 
effect.  Compound- wound  machines  have  two  methods 
of  connecting,  viz.:  long  shunt  and  short  shunt.  In 
the  long-shunt  connection,  as  given  in  Fig.  17,  the 
shunt  winding  is  connected 
to  the  machine's  terminals 
and  hence  outside  the  series 
windings.  When  the  line 
switch  is  open  the  E.  M.  F. 
is  that  due  to  the  shunt  field 
plus  a  small  amount  due  to 
the  series  field.  In  the  short-  _. 

shunt    connections  of  Fig.    18 

the  shunt  winding  excludes  the  series,  and  the  latter  is 
entirely  idle  on  open  circuit.  The  two  types  require 
slightly  different  calculations  in  designing,  but  are  identi- 


— 0 — W} — 


32  TESTING    OF    DYNAMOS    AND    MOTORS. 

cal  in  action.     If  the  coils  are  properly  proportioned  the 
gain  in  the  series  field,  as  the  load  goes  on,  balances  the 
loss  in  the  shunt  field,  and  from  no  load  up  to  the  point 
,"/o^~>1  where    the   field    cores    become 

saturated,  the  terminal  E.  M.  F. 
remains  constant.  Any  further 
increase  in  load  will  lower  the 
E.  M.  F.,  because  not  only  does 
armature  reaction  set  in,  but  the 
increased  current  raises  the 
/  7?  loss,  without  appreciably 
strengthening  the  already  saturated  field. 

There  are  three  sources  of  loss  in  a  dynamo  or  motor, 
viz.,  mechanical,  electrical,  and  magnetic.  The  first 
comprises  the  frictional  losses  in  the  bearings,  brushes, 
and^elts.  In  the  second  are  found  the  I*R  losses  of 
armature  and  fields;  their  value  is  easily  calculable,  and 
in  well-designed  machines  is  not  large.  The  third  source 
of  loss  is  in  the  magnetic  circuit,  and  the  component 
factors  are  not  so  easily  separated.  In  general,  magnetic 
losses  are  due  to  a  change  in  the  quantity  or  direction  of 
magnetic  flow  setting  up  molecular  friction  or  inducing 
currents;  in  either  case  the  product  is  heat,  and  rep- 
resents lost  energy.  Thus  the  field  current  fluctuates 
within  narrow  limits,  and  the  iron  accordingly  gains  or 
loses  magnetization,  molecular  friction  ensues,  and  heat 
is  produced.  In  the  pole-heads  this  action  is  strongest, 
and  is  most  marked  in  machines  having  lug  armatures. 
The  lugs  are  concentrators  of  lines  of  force,  and  sweep 
across  the  polar  faces  very  much  the  same  as  one  might 
do  with  a  brush.  The  effect  can  even  be  detected  with 
the  hand  by  feeling  the  polar  horns  on  a  machine  in  ser- 


ELEMENTS    OF    THE    DYNAMO. 


33 


FIG.   19. 


vice.  On  a  dynamo  the  leading  horns,  while  on  a  motor 
the  trailing  horns,  become  heated.  The  most  serious 
magnetic  loss  is  due  to  eddy  currents  in  the  armature  core 
and  pole-heads.  Considering  the  armature  to  be  made  up 
of  concentric  cylinders  of  metal,  as  indicated  in  Fig.  19, 
it  is  virtually  composed  of  layers  of  con- 
ductors on  closed  circuit,  revolving  in  a 
powerful  magnetic  field.  Were  the  ar- 
mature core  solid,  in  it  would  flow  a  cur- 
rent giving  rise  to  much  heating  of  the 
core  itself.  Fortunately  this  trouble 
can  be  avoided  by  building  up  the  ar- 
mature of  thin  sheet  iron  plates.  The 
resistance  of  so  many  joints  prevents  any  serious  flow  of 
current.  _t  Another  magnetic  loss  is  due  to  a  modification 
of  the  magnetic  field  by  the  armature.  The  armature  is 
a  huge  electromagnet  with  its  poles  at  the  neutral  line, 
and  therefore  with  its  poles  approxi- 
mately at  right  angles  to  those  of  the 
field.  The  armature  tends  to  send  its 
lines  of  force  up  and  down  like  the 
line  a  b,  Fig.  20,  while  the  field 
would  send  its  own  straight  across, 
like  the  line  c  d.  Either  alone  would 
have  its  way,  but  the  resultant  of  the 
forces  is  in  the  direction  of  the  dotted 
line  f  /,  and  this  marks  the  diameter  on  which  are  set 
the  brushes  of  a  bipolar  machine.  This  armature  effect  is 
known  as  cross-induction,  and  its  effect  upon  the  position  of 
the  neutral  point  is  plainly  seen  in  the  necessity  of  shifting 
the  brushes  on  most  machines.  Now  the  shifting  of  the 
neutral  line  is  in  such  a  direction  that  like  poles  of  field 


F|G   20 


34  TESTING    OF    DYNAMOS    AND    MOTORS. 

and  armature  are  brought  nearer  together,  and  the  arma- 
ture ampere-turns  strive  to  force  a  flow  of  magnetism 
through  the  same  magnetic  circuit  as  the  field  ampere- 
turns,  but  in  the  opposite  direction.  Hence  the  arma- 
ture exerts  a  demagnetizing  effect,  with  the  result  that 
the  field  current  must  be  increased  to  make  it  up,  and 
this  means  an  additional  P  R  loss  in  the  field  wind- 
ing. This  effect  is  called  back  induction,  and  in  machine 
design  allowance  is  made  for  it  by  providing  additional 
field  winding.  The  lamination  relieves  the  magnetic 
troubles  as  far  as  they  can  be  relieved.  In  alternators 
the  pole-heads  are  laminated  as  well  as  the  armature 
cores. 

The  extent  to  which  these  many  losses  are  done  away 
with  determines  the  Efficiency  of  the  machine.  If  we  could 
get  out  of  a  machine  all  the  work  which  we  put  into  it, 
we  could  say  the  machine  had  an  efficiency  of  one  hun- 
dred per  cent.  But  such  a  thing  is  impossible,  as  in  any 
machine  part  of  the  power  given  to  it  is  always  wasted  in 
friction,  and  in  electrical  machines  we  must  add  to  this 
electrical  and  magnetic  losses.  Of  the  energy  given  to 
an  electrical  machine  we  can  reclaim  only  a  fraction, 
and  the  value  of  this  fraction  expresses  the  efficiency. 
The  term  efficiency  is  accepted  in  either  of  two  senses — 
Electrical  Efficiency  or  Commercial  Efficiency.  The 
electrical  efficiency  is  gotten  by  dividing  the  electrical 
energy  available  by  the  total  electrical  energy  generated 
by  the  machine,  and  takes  into  account  only  the  72  R 
losses.  The  commercial  efficiency  is  had  by  dividing  the 
total  energy  gotten  out  of  the  machine  by  the  total  energy 
put  into  it,  and  includes  all  the  various  losses.  Calling 
£L  and  7L  the  E.  M.  F.  and  current  respectively  of  the 


ELEMENTS  OF  THE  DYNAMO.  35 

line,  and  £r  and  7T  the  total  E.  M.  F.  and  current  of  the 
machine,  the  expression  for  electrical  efficiency  is 

Elect.  E.  =    ^L/I- 


or  watts  produced  divided  by  watts  taken  out.  The  special 
form  which  this  expression  takes  depends  upon  the  type 
of  the  machine.  The  formula  for  commercial  efficiency  is 

Com.  Eff.  =  y*  , 

where  w  is  the  energy  taken  out  electrically  and'  \V 
is  the  energy  put  in  mechanically  by  the  steam  engine. 
To  compare  them  they  must  be  reduced  to  the  same 
units. 


CHAPTER  II. 

ELEMENTS   OF    THE    MOTOR. 

IF  the  terminals  of  a  dynamo  be  connected  to  a 
source  of  current,  the  machine  becomes  a  motor,  trans- 
forming the  electrical  energy  drawn  from  the  line  into 
mechanical  energy  at  the  pulley.  Motors  are  classi- 
fied in  the  same  manner  as  are  dynamos,  and  the  same 
general  principles  of  dynamo  design  and  regulation 
enable  us  to  anticipate  and  explain  motor  action  under 
like  conditions.  The  fundamental  fact  which  makes 
electric  motors  possible,  is  the  following:  If  a  conductor 
carrying  a  current  be  placed  in  a  magnetic  field,  it  will  be 
impelled  in  such  a  direction  that  the  E.  M.  F.  induced  in  it  by 
this  motion  will  oppose  the  flow  of  current  already  existing 
in  the  conductor.  This  law  states  that  there  will  be 
motion,  and  also  gives  the  direction  of  this  motion.  The 
conditions  to  be  fulfilled  are  then,  a  magnetic  field,  con- 
ductors in  this  field,  carrying  a  current  and  free  to 
move.  The  same  machine  that  is  used  as  a  dynamo  fills 
these  requirements  of  the  motor.  Let  us  take  a  machine 
with  separately  excited  or  permanent  fields,  and  investi- 
gate its  action.  If  the  brushes  be  connected  to  a  battery 
a  current  flows  through  the  armature.  Let  N  and  6" 
(Fig.  21)  be  the  fields,  and  A,  the  armature.  With 
fields  the  same  as  when  the  machine  is  used  as  a 
dynamo,  let  the  positive  brush  (*.  e.,  the  positive 


ELEMENTS   OF    THE    MOTOR. 


37 


dynamo  brush)  be  connected  to  the  positive  pole  of  the 
battery.  The  direction  of  the  current  will  then  be  the 
reverse  to  what  it  was  in  the  dynamo,  and  the  direction 
of  rotation  of  the  armature  the  same  as  that  of  the 
dynamo,  for  the  E.  M.  F.  thus  induced  will  tend  to  set 
up  a  current  opposed 
to  the  existing  one. 
If,  then,  a  machine  be 
so  connected  as  to 
have  its  fields  of  a 
given  polarity,  and  its 
brushes  of  constant 
polarity,  /.  f. ,  the 
same  brush  always 
positive  on  the  dy-  Fig.  21. 

namo,    and    when    on 

a  motor  connected  to  the  positive  side  of  the  circuit — 
the  direction  of  rotation  is  the  same  whatever  be  the 
machine's  nature.  In  other  words,  if  in  the  two  cases 
the  field  current  flow  in  the  same  direction  while  the 
armature  curfent  flows  in  opposite  directions,  the  direc- 
tion of  rotation  will,  in  the  motor,  be  the  same  as  in  the 
dynamo.  This  explains  why  shunt  machines  run  in  the 
same,  but  series  machines  in  opposite  directions,  when 
their  nature  is  changed  but  the  connections  left  the 
same. 

The  E.  M.  F.  to  which  we  have  referred  as  oppos- 
ing the  existing  flow  of  armature  current,  is  called  the 
Counter  E.  M.  F.y  and  is  of  fundamental  importance  in 
motor  theory.  It  is  a  source  of  confusion,  unless  properly 
understood,  and  some  misguided  inventors  have  vainly 
endeavored  to  devise  motors  without  any  C.  E.  M.  F., 


38  TESTING    OF    DYNAMOS    AND    MOTORS. 

supposing  its  presence  to  be  a  detriment  to  the  machine. 
A  very  little  consideration  shows  that  whether  a  help  or 
a  hindrance,  its  presence  is  unavoidable,  and  an  arma- 
ture wound  to  have  no  C.  E.  M.  F.,  would  have  no  power 
of  motion;  for,  to  deprive  an  armature  of  its  counter  is 
to  deprive  it  of  its  ability  to  generate  voltage  when  run 
as  a  dynamo  in  a  magnetic  field,  its  C.  E.  M.  F.  being 
nothing  other  than  the  dynamo  property  of  a  motor.  To 
construct  such  an  armature  its  conductors  must  be  so 
wound  that  any  tendency  of  one  conductor  to  generate  an 
E.  M.  F.  is  met  by  the  tendency  of  some  other  conductor 
with  which  it  is  in  series  to  generate  an  equal  but  oppo- 
site E.  M.  F.  To  run  such  an  armature  as  a  motor  would 
be  impossible,  because  the  flow  of  current  through  it 
would  be  so  disposed  that  one-half  the  conductors  would 
attract  a  pole  piece,  the  other  half  repel  it;  and  the 
equal  but  opposite  tendencies  to  rotate  the  armature 
would  neutralize  each  other  and  there  would  be  no 
motion  at  all.  In  every  conductor  moving  in  a  magnetic 
field  an  E.  M.  F.  is  produced;  that  this  E.  M.  F.  must 
oppose  the  motor  current,  can  be  easily  see"n,  for  suppose 
it  to  assist  the  current:  as  the  speed  and  induced  E.  M.  F. 
increased,  the  current  and,  with  it,  the  power  would 
increase  also.  That  is  to  say,  the  motor  would  con- 
tribute to  its  own  driving  power,  and  would  do  the  same 
work  at  the  pulley  with  less  and  less  demand  upon  the 
line,  and  finally,  the  line  supply  might  be.  dispensed  with 
altogether.  This  is  absurd,  and  theory  as  well  as  experi- 
ence indicates  that  the  induced  E.  M.  F.  will  oppose  the 
impressed  or  line  E.  M.  F.  Furthermore,  as  we  shall 
learn,  the  higher  this  C.  E.  M.  F.  becomes,  the  greater 
is  the  efficiency  of  the  motor. 


ELEMENTS   OF    THE    MOTOR.  39 

The  power  given   to  the  motor  equals  the  product  of 
the  armature  current  and  the  impressed  E.  M.  F.  or, 

Watts  consumed  =  E  /a. 

This  is  divided  into  two  parts:  that  which  is  wasted  as 
heat  in  the  armature,  and  that  which  is  transformed  into 
mechanical  power,  and  does  work  at  the  pulley.  We 
may,  then,  write:  ll'atts  consumed  --  watts  transformed 
-f-  watts  wasted  in  heat.  The  heat  waste  is  due  to  mag- 
netic and  electric  losses,  but  until  we  take  up  commercial 
efficiency  the  latter  only  will  be  considered,  and  these  in 
the  armature  alone.  The  electric  loss  we  know  to  equal 
/"*'  /•„  =  /  x  the  E.  M.  F.  necessary  to  urge  the  given  cur- 
rent through  the  ohmic  resistance  t\  of  the  armature, 
and  since  the  total  watts  consumed  =  E  /a,  the  watts 
transformed  must  equal  the  difference  between  the  two, 
or,  in  other  words,  is  the  product  of  the  current  by  that 
part  of  the  impressed  E.  M.  F.  not  expended  in  overcom- 
ing ohmic  resistance.  We  may  then  write, 

E  /a  =  watts  transformed  -\-  /9a  rtt,  (1) 

whence, 

Watts  transformed  =  Et\  —  /*a  ra  =  /a  (E —  ia  ra),    (2) 
Now, 

E  -  e 

'•=-7—' 

where  e,  is  the  C.  E.  M.  F. ;  clearing  of  fractions  we  get 
/a  ra  =  E  —  e,  and  substituting  this  in  the  expression  for 
watts  transformed,  we  have, 

Watts  transformed  =  /a  (E  —  E  +  e)  =  /a  *,        (3) 
which    shows    that    (barring    other    losses)    the    watts 
transformed  into  mechanical  power    equal    the  product 
of   the    current    and    the   C.   E.    M.   F.       The   equation 
E  —  e  —  ;a  ru  shows  how  far  the  C.   E.  M.  F.  falls  below 


TESTING    OF    DYNAMOS    AND    MOTORS. 


the  impressed,  and  that  the  difference  is  smaller  the  lower 
the  armature  resistance.  If  it  were  possible  to  construct 
an  armature  of  zero  resistance,  e  would  equal  £,  and  for 
all  loads  the  electrical  efficiency  would  be  100  per  cent. 
The  expression  for  the  electrical  efficiency  is 

watts    transformed        e  za         e  ,^. 

^e  ~       watts  consumed       ~  ~E7&  ~  ~E* 

showing   the   efficiency  to   increase   as  the   C.  E.  M.  F. 

approaches  the   impressed.      Two   questions  here  arise 

(a)    With  what  current  does  a  motor  do  most  work  in 


d 

FIG.  22. 

least  time?  (b)  With  what  current  does  it  work  most 
efficiently?  The  first  may  be  solved  as  follows:  Sub- 
stituting W  for  watts  transformed,  in  equation  (2),  we 
have, 


whence, 


E± 


(5) 


and  the  question  is,  for  what  value  of  ia  will  Wbe  a  max- 
imum. Let  us  represent  the  relation  between  /a  and  W 
by  a  diagram,  Fig.  22,  in  which  the  values  of  £Fare  laid 
off  on  the  vertical  scale,  and  the  corresponding  values 


ELEMENTS    OF    THE    MOTOR.  4! 

of  /a  on  the  horizontal  scale.  If  in  equation  (5)  we 
begin  at  zero,  and  give  gradually  increasing  values  to 
//••',  and,  with  the  knowledge  of  E  and  ;-a,  work  out  the 
corresponding  values  of  /a,  the  points  on  4,  and  \V,  will, 
if  projected  as  indicated  by  the  dotted  lines,  give  a  series 
of  intersections  whose  path  will  be  the  curve  a  c  b.  For 
every  value  of  W  there  will  be  two  values  of  /a.  "\Vhen 
IV  =  o,  equation  (5)  becomes, 


and  the  two  values  of  /a  are, 

2  E  _     F^ 
/n  ~  2   r.  -       rl 
and 

E-  E 

'n   =  -          -   =   O. 

The  o  value  corresponds  to  open  circuit,  and  is  repre- 
sented by  the  point  a\  the  value 

E_ 
r* 

means  that  the  armature  is  blocked  so  that  it  cannot 
turn,  and  is  represented  by  the  point  b.  At  c  the  two 
values  of  JFare  identical,  and  here  we  find  the  maximum 
value  for  W.  This  condition  is  fulfilled  when 


and  from  this  we  get  by  equation  (5) 

E 


42  TESTING    OF    DYNAMOS    AND    MOTORS. 

or  one-half  its  maximum  value.     But 

E-e 

'«  =  —> 

therefore 

_E_     _  E-e 

2~^  "         r*      ' 

and  e  =  1/2  E,  so  that  the  expression  for  electrical  effi- 
ciency becomes 

1/2   E  fp\ 

V*  =  J-£-  =  5°^  (6) 

That  is  to  say,  a  motor  works  fastest  at  an  electrical 
efficiency  of  50^.  As  a  rule  motors  are  designed  for 
a  much  higher  efficiency  than  50  $,  and  to  run  them  at  so 
low  a  figure  might  require  excessive  current  overload. 
For  a  long  time  this  efficiency  of  maximum  activity  was 
mistaken  for  the  highest  attainable  efficiency,  and  it  was 
declared  that  motor  efficiency  could  not  exceed  this 
value.  This  is  manifestly  an  error,  as  electrical  efficien- 
cies of  90  fc  and  95  %  are  commonly  attained  in  motors 
of  to-day.  On  the  other  hand,  as  the  efficiency  in- 
creases, work  per  minute  decreases,  and  to  meet  the 
demands  of  traffic  it  is  found  more  profitable  to  run  at 
higher  outputs  per  minute  and  lower  efficiencies  (80  $ 
to  90  #). 

Thus  far  we  have  had  to  do  with  electrical  efficiency 
only,  and  have  confined  our  attention  to/2./?  losses.  The 
commercial  efficiency  takes  into  account  frictional  and 
other  losses,  and  is  of  more  practical  importance.  It  can 
be  found  as  follows:  Ascertain  at  what  speed  the  arma- 
ture turns  when  loaded,  and  call  the  current  flowing,  i&. 
Next  determine  the  current,  *'a,  necessary  to  turn  the 


ELEMENTS    OF    THE    MOTOR.  43 

armature  free  and  at  the  same  speed.  With  load,  the 
current  doing  useful  work  is,  /a  —  /"'a,  and  the  expression 
for  commercial  efficiency  becomes, 


- 

VC     - 

Remembering  that, 


E  -e 


where  ra  is  the  armature  resistance,  we  get, 

E  -  e        ., 

<       >\       ~  l  a  _  e  E  -  e*  -  c  ra  /  \ 
7/c  -  E  ~~E^rr~  E*-tE 


e  (E-e  -  ra/'a) 

E  (E  -  c) 
The  commercial  efficiency  for  maximum  activity,  is, 


The  maximum  commercial  efficiency  is  a  more  complex 
question,  as  it  involves/",  and  ra,  as  well  as  the  C.  E.  M.  F. 
If  in  equation  (7),  ra  and  E  be  given,  the  value  of  /a,  cor- 
responding to  maximum  efficiency,  can  be  found.*  The 
condition  is  that 

E  -  e  =  ^^£T*  (8) 

or  that  _ 

e  =  E  - 


To  illustrate:  a  motor   has  an   impressed  E.  M.  F.  (E) 

*  For  a  full  discussion,  see  Kapp's  Electrical  Transmission  of  Energy, 
4th  ed.,  pages  155  to  157. 


44  TESTING    OF    DYNAMOS    AND    MOTORS. 

of  no  volts.  Running  free,  the  armature  takes  (/'a)  5 
amperes.  Armature  resistance  (ra)  =  .35  ohm.  What 
is  the  maximum  commercial  efficiency?  From  equa- 
tion (8) 

no  —  e  —  ^.35  X  no  x  5 
whence,    e  —  96    volts  =  C.  E.  M.  F. 
Now, 

E  -  e          14 

/a  =  -  =  —  =  40  amperes. 

'a  -35 

Therefore  the  maximum  electrical  efficiency  is 
c          96 

and  maximum  commercial  efficiency,  by  equation  (7),  is 

/7C  =  76.6  %. 
While  the  HP  available  at  the  pulley  is, 

HP  =  5.1. 

The  reason  the  efficiency  is  so  low  is  that  the  armature 
resistance  is  so  high;  let  ra  be  lowered  to  .2  ohm,  and 
we  have 


HP  =  7.1. 

Torque. — It  is  a  familiar  fact  that  between  unlike  poles 
of  neighboring  magnets  a  force  of  attraction  exists.  If 
the  magnets  be  crossed  at  right  angles  to  each  other,  and 
one  of  them  be  free  to  rotate,  it  will  do  so  until  the 
unlike  poles  are  as  near  together  as  possible.  The 
relation  between  the  armature  and  fields  of  a  motor  is 
similar  to  this.  The  armature  is  an  electromagnet  with 


ELEMENTS    OF    THE    MOTOR.  45 

its  poles  midway  between  those  of  the  field,  resulting 
in  an  attraction  of  unlike  poles  of  armature  and  field. 
As  soon  as  a  coil  reaches  a  position  where  it  would  natu- 
rally remain  at  rest,  its  current  is  reversed,  its  former 
relative  polarity  restored,  and  continuous  rotation  ensues. 
The  force  with  which  the  armature  tends  to  turn  is  meas- 
ured by  the  product  of  the  pole  strengths  of  field  and 
armature,  and  will  vary  as  this  product  varies.  If  we  can 
assume  the  field  poles  to  remain  constant,  the  turning 
force  will  vary  as  the  armature  current  varies.  This  all- 
important  turning  force  is  called  Torque,  and  its  value 
has  been  mathematically  proven  to  be, 


where  AT  is  the  total  number  of  lines  of  force  crossing  the 
air  gap,  C  is  the  number  of  conductors  on  the  armature, 
and  /a  equals  the  current  in  each  conductor  or  one-half  the 
current  in  the  armature.  In  so  many  words  torque  or 
twisting  force  is  nothing  more  than  a  force  acting  with  a 
leverage  in  the  same  way  as  a  man,  with  the  force  of  his 
own  weight,  can,  with  a  crowbar  as  a  lever,  move  a  weight 
many  times  greater  than  his  own.  In  the  case  of  an 
armature  the  force  acting  is  the  force  of  attraction 
between  the  armature  current  and  the  lines  of  force  of 
the  field,  and  the  leverage  with  which  it  acts  is  the  dis- 
tance from  the  centre  of  the  armature  core  to  the  con- 
ductors. If  the  force  of  attraction  is  measured  by  the 
product  of  field  lines  and  armature  current,  it  is  obvious 
that  increasing  either  of  these  will  increase  the  turning 
power  of  the  armature  :  this  the  above  formula  tells  us, 
for  if  N  or  C  or  /a  is  increased,  the  value  of  the  frac- 


46  TESTING    OF    DYNAMOS    AND    MOTORS. 

tion  is  also.  The  work  put  into  an  armature  can 
be  expressed  as  equal  to  the  speed  of  the  arma- 
ture multiplied  by  its  torque,  or  W  —  6.28  x  n  X  T. 
If  the  amount  of  work,  IV,  is  kept  the  same,  as  the  speed 
rises  the  torque  falls,  and,  could  the  armature  attain  the 
impossibility  of  having  e  •=.  E,  there  would  be  no  current, 
hence  no  torque;  there  would  be  no  work  put  into  the 
motor  and  none  taken  out.  When  the  current  is  first 
turned  into  a  motor  the  torque  is  greatest,  for  torque 
depends  upon  the  current,  and  just  before  starting,  when 
the  C.  E.  M.  F.  is  zero,  the  current  is  greatest  for  a 
given  mechanical  load.  When  running  free  the  arma- 
ture speed  is  highest,  and  e  approaches,  but  never  reaches, 
E.  One  sometimes  hears  the  statement  that  the  C.  E. 
M.  F.  could  only  equal  the  impressed  were  the  speed 
infinite:  this  is  an  error,  for  with  any  field  strength  an 
infinite  armature  speed  would  produce  an  infinite  E.M.F. 
The  formula  for  the  E.  M.  F.  of  a  dynamo  is  E  =  N  C 
n  -4-  100,000,000.  Since  C.  E.  M.  F.  is  only  the  dynamo 
E.  M.  F.  of  a  motor  for  that  speed  and  excitation,  this 
expression  is  good  for  the  C.  E.  M.  F.  of  a  motor,  and 
leaving  off  the  100,000,000,  which  serves  to  reduce 
C.  G.  S.  units  to  volts,  we  can  write  as  the  C.  E.  M.  F. 
of  any  motor  e  —  N  C  n.  Dividing  both  sides  of  this 
equation  by  the  same  term  does  not  alter  its  value,  so  we 
divide  by  N  C  to  get  n  on  one  side  alone ; 


leaving 

n  = 


ELEMENTS    OF    THE    MOTOR.  47 

an  expression  for  speed,  which  does  not  contain  torque, 
showing  the  torque  to  be  independent  of  the  speed. 
Again,  this  formula  shows  that  if  N  is  kept  the  same, 
the  field  a  series  field,  or  separately  excited,  the  denomi- 
nator of  the  fraction  becomes  a  fixed  number,  or, 
as  we  say,  a  constant,  and  the  value  of  the  fraction  can 
only  be  varied  by  varying  the  numerator  e.  In  other 
words  Con  any  given  armature  is  a  fixed  number;  if  we 
fix  N,  then  n  depends  upon  e  and  varies  with  it  in  direct 
proportion,  /*.  <-.,  the  speed  of  any  motor  armature  is  pro- 
portional to  its  C.  E.  M.  F.,  and  if  the  armature  and  field 
resistances  are  low,  ;/  is  also  proportional  to  /:,  the 
impressed  or  line  voltage.  If  these  resistances  are  high, 
the  loss  of  voltage  through  them  cannot  be  neglected, 
and  n  cannot  be  said  to  vary  directly  as  Ey  but  directly 
as  E  less  the  internal  loss,  which  difference  is  e,  the 
C.  E.  M.  F.  In  other  words  if  with  an  impressed 
E.  M.  F.,  E  —  500,  we  get  a  speed  ;/  =  200,  a  voltage  of 
1,000  will  not  give  a  speed  of  400,  and  to  know  what  speed 
it  will  give  we  must  assume  same  current  value  at  which 
both  tests  are  to  be  run,  and  know  the  value  of  internal  re- 
sistance. Let  /  =  30  amperes,  and  let  r  —  10  ohms  (an 
exaggerated  case).  In  the  first  test,  then,  the  internal 
loss  of  voltage  =  /  r  =  10  X  30  =  300,  and  the  speed  of 
200  revolutions  is  really  due  to  200  volts,  or  i  revolution 
per  volt.  In  the  second  case  the  current,  /,  and  re- 
sistance, r,  are  the  same  as  before,  so  the  volts  lost,  = 
i  r,  must  be  the  same,  or  300.  This  leaves  700  volts  to 
cause  motion,  and  at  i  revolution  per  volt  this  would 
correspond  to  a  speed  of  700  revolutions.  If  we  assume 
r  =  $y$  ohms,  the  loss  in  both  tests  is  100  volts,  and  rais- 
ing E  from  500  to  1,000  raises  n  from  200  to  450,  while  if 


48  TESTING    OF    DYNAMOS    AND    MOTORS. 

r  =  i  ohm,  n  will  increase  only  from  200  to  412,  or  very 
nearly  proportionally  to  E.     Now  in  the  formula 


anything  which  increases  the  denominator  of  the  fraction 
decreases  the  value  of  the  fraction,  /.  ^.,  decreases  n;  this 
shows  us,  then,  that  if  we  increase  N  or  C,  keeping  e  the 
same,  we  will  decrease  ;z,  and  vice  versa.  Theory  here 
predicts  what  is  every  day  practiced,  viz.  :  that  to  speed 
up  a  motor  we  weaken  its  field.  The  physical  explanation 
of  this  fact  is,  that  in  weakening  the  field  the  armature 
cannot  generate  as  high  a  C.  E.  M.  F.,  hence  a  larger 
current  flows  through  the  armature  and  its  speed  rises. 
That  weakening  the  field  on  a  motor  will  raise  the  speed 
is  true  of  all  recent  types  of  motor,  but  it  is  a  remarkable 
fact  that  where  a  motor  runs  at:  an  efficiency  of  less  than 
50  $,  strengthening  the  field  will  raise  the  speed, 
and  weakening  the  field  lower  the  speed.  The  explana- 
tion is  as  follows:  For  a  motor  to  run  at  normal  load 
with  an  electric  efficiency  of  less  than  50  $,  means 
that  the  internal  resistance  must  be  so  high,  that  of  the 
total  E.  M.  F.  applied  to  the  motor  terminals  over  half 
is  wasted  in  urging  the  current  through  this  resistance, 
and  less  than  half  is  left  to  cause  motion.  To  run  a 
motor  at  a  certain  speed  requires  a  certain  amount  of 
work,  and  to  run  it  above  or  below  this  speed  requires 
more  or  less  work  respectively;  now  the  effect  of  weaken- 
ing the  field  is  to  lower  the  C.  E.  M.  F.  and  to  raise  the 
current  through  the  armature,  and  this  ought  to  raise  the 
speed,  but  if  the  resistance  is  high,  the  increased  current 
causes  so  large  a  loss  in  voltage  that  although  the  motor 
draws  more  energy  from  the  line  than  before,  there  is  less 


ELEMENTS    OF    THE    MOTOR.  49 

available  for  motion,  so  that  with  decreased  energy  the 
motor  must  run  slower.  An  example  will  help  us.  Let 
/  =  30,  and  r  =  10,  and  at  a  given  field  strength  let  there 
be  a  given  speed.  £,  as  before,  =  500.  Volts  lost  = 
i  r  —  30  x  10  =  300.  Useful  volts  =  500  —  300  =  200. 
If  useful  E  =  200,  and  /  =  30,  the  watts  expended  in 
motion  —  E  (useful)  x  / '  =  200  x  30  =  6,000  watts. 
Now  let  the  field  be  weakened  till  /  =  35,  then,  volts 
lost  =  /  r  =  35  x  10  =  350.  Useful  volts  =  150,  and 
useful  energy  =  useful  £  x  t  =  150  x  35  =  5»25°»  as 
against  6,000,  so  the  speed  must  fall.  If  r  =  i,  then, 
lost  and  useful  volts  at  30  amperes  are  30  and  470  respec- 
tively, and  at  35  amperes,  35  and  465,  and  the  useful 
energy  at  30  amperes  is  14,100  watts,  while  that  at  35 
amperes  is  16,275.  So  the  effect  in  one  case  is  to  decrease 
the  useful  energy  and  in  the  other  to  increase  it,  the 
speed  varying  accordingly.  Upon  this  fact  depends  the 
action  of  the  Thomson  wattmeter. 

The  question  of  speed  regulation  is  important  in 
motor  work.  Viewed  from  this  standpoint  there  are 
two  general  classes  of  motors,  (i)  traction  and  (2)  sta- 
tionary motors.  In  the  first  class  a  large  initial  torque  is 
required  and  variable  speed  under  easy  control.  The  sec- 
ond class  in  almost  all  cases  calls  for  constant  speed  under 
all  loads.  Another  classification  regards  the  nature  of 
the  circuit  upon  which  the  motor  is  to  be  used  :  (i)  motors 
on  constant  potential  (C.  P.)  mains,  (2)  constant  current 
motors.  In  the  first  all  the  motors  are  in  parallel,  /.  e., 
the  total  current  flowing  divides  among  the  motor  circuits, 
and  each  circuit,  supplied  with  the  full  line  voltage,  is 
independent  of  the  other  circuits;  whereas  with  constant 
current  motors  there  is  but  one  circuit,  and  the  total  cur- 


5<D  TESTING    OF    DYNAMOS    AND    MOTORS. 

rent  passes  successively  out  of  one  motor  into  another,  s<? 
that  any  unusual  action  on  the  part  of  one  may  affect  all. 

The  usual  classification  is  based  upon  the  style  of  field 
excitation: — (i)  separately  excited,  (2)  series,  (3)  shunt, 
(4)  compound.  The  series  motor  is  represented  in  street- 
car work;  the  shunt  motor  in  stationary  work,  where 
small  speed  variations  are  unimportant.  Compound- 
wound  motors  are  used  where  automatic  speed  regula- 
tion is  desired  under  variations  of  load.  The  behavior 
of  each  type  may  be  briefly  outlined  as  follows: 

Separately  excited  motors  have  a  field  independent  of 
the  armature  current.  As  the  load  increases  so  does  the 
armature  current,  and  with  it  the  torque,  while  the  speed 
gradually  falls  off.  As  the  armature  reaction  increases  it 
may  be  necessary  to  give  the  brushes  a  negative  or  back- 
ward lead,  thus  increasing  the  demagnetizing  effect  upon 
the  field.  This  weakening  of  the  field  tends  to  increase 
the  speed,  so  that  such  a  motor  with  strong  armature  re- 
action will  regulate  fairly  well  for  speed  within  certain 
limits.  Whether  it  is  necessary  or  not  to  shift  the 
brushes,  the  demagnetizing  effect  of  the  armature  in- 
creases as  its  current  does,  for  it  radiates  more  lines  of 
force,  and  care  must  be  taken  that  the  armature  does  not 
overpower  the  fields  and  reverse  the  direction  of  rota- 
tion. Warning  of  this  is  given  by  destructive  sparking 
and  an  abnormal  increase  in  the  current. 

The  series  motor  is  adapted  to  railway  'sevice  where  it 
is  often  necessary  to  start  a  heavy  car  on  a  grade.  On 
closing  the  circuit  the  field  and  armature  currents  rise 
immediately  to  a  value  controlled  by  the  resistance  in 
circuit,  and  the  torque  is  very  great.  Sometimes  this 
resistance  is  in  a  separate  coil  alone,  and  sometimes  the 


ELEMENTS    OF    THE    MOTOR.  51 

fields  are  in  sections  whose  resistance,  when  in  series,  is 
almost  sufficient  to  limit  the  initial  current  to  a  safe 
value.  With  fields  in  series  their  ampere-turns  are 
greatest  and  provide  the  torque  so  much  needed  at 
starting.  As  the  motor  acquires  speed  the  current 
decreases,  on  account  of  the  C.  E.  M.  F.,  and  with  it 
the  torque,  to  which  it  remains  very  nearly  propor- 
tional. Speed  regulation  is  obtained  either  by  throw- 
ing the  field  coils  from  simple  series  to  different 
combinations  of  series  ~ncl  multiple,  as  in  the  old  Edison 
system,  or  by  cutting  out  or  shunting  part  of  the  field 
ampere-turns  as  in  more  recent  types.  In  the  method 
of  commutatecl  fields  a  special  switch  is  used,  by  the 
turning  of  whose  handle  the  different  combinations  are 
effected.  In  the  more  recent  methods  of  control,  the 
car  is  started  with  motors  in  series,  and  by  steps  they  are 
thrown  into  multiple,  thereby  increasing  not  only  the 
speed  control,  but  economy  in  service — a  point  to  be 
considered  later. 

Shunt  motors  have  a  wide  scope  of  work,  being  very 
generally  used  for  isolated  motor  work  and  for  shop 
service  on  constant  potential  lighting  mains.  The  fields 
are  so  connected  that  they  are  charged  before  the  arma- 
ture circuit  is  closed.  These  precautions  are  necessary 
because  the  field  in  shunt  with  the  armature  is  of  high 
resistance  and  self-induction,  so  that  not  only  is  the 
armature  virtually  a  short  circuit  across  it,  but  even 
under  the  best  conditions  its  induction  would  prevent  its 
acquiring  full  strength  immediately.  Were  no  precau- 
tion taken,  the  switch  closed  across  field  and  armature 
at  the  same  time,  and  the  machine  left  to  pick  up  a 
C.  E.  M.  F.  as  best  it  could,  trouble  would  ensue  before 


52  TESTING    OF    DYNAMOS    AND    MOTORS. 

the  rush  of  current  could  be  checked.  Even  when  the 
fields  are  connected  above  the  switch  so  as  to  give  the 
required  field  strength  before  closing  the  armature  cir- 
cuit, it  is  necessary  to  close  the  armature  circuit  through 
a  resistance,  and  then  to  cut  the  resistance  out  after  the 
speed  is  up. 

Variations  of  load  on  a  shunt  motor  fed  from  constant 
potential  mains  produce  slight  speed  variations,  the 
regulation  being  analogous  to  regulation  for  E.  M.  F. 
in  a  shunt  dynamo  run  at  constant  speed.  The  action  is 
as  follows:  the  work  depends  upon  the  product  of  the 
speed and  torque,  and  the  torque  depends  upon  the  product 
of  the  pole  strengths  of  armature  and  field.  The  arma- 
ture pole  strength  depends  upon  the  current,  and  in 
order  to  increase  this  the  C.  E.  M.  F.  must  be  lowered; 
this  requires  a  decrease  in  either  speed  or  field  strength. 
As  a  matter  of  fact  both  effects  take  place.  The  added 
load  on  the  pulley  slows  up  the  armature,  the  C.  E.  M.  F. 
falls,  and  a  larger  current  flows.  With  the  increased  cur- 
rent the  armature  reaction  is  increased,  and  the  weaken- 
ing of  the  field  raises  the  speed.  The  decrease  in  speed, 
however,  is  not  entirely  compensated  for,  and  the  motor 
gradually  slows  down.  Still  another  effect  which  pre- 
vents compensation  from  being  complete,  is  that  the  lost 
voltage  in  the  armature  increases,  thus  decreasing  the 
E.  M.  F.  effective  in  producing  rotation.  Armatures  of 
no  resistance  would,  aside  from  their  own  reaction,  regu- 
late for  constant  speed  on  a  motor  just  as  they  would 
for  constant  E.  M.  F.  on  a  dynamo.  Hand  regulation 
may  be  resorted  to,  either  in  raising  the  impressed 
E.  M.  F.,  or  in  weakening  the  fields. 

This  gradual  weakening  of  the  field  as  load  increases 


ELEMENTS   OF    THE    MOTOR.  53 

is  automatically  attained  in  the  differentially  connected 
compound-wound  motor.  In  this,  the  shunt  winding  is 
made  strong  enough  to  always  control  the  polarity  of  the 
field,  and  the  series  winding,  instead  of  assisting  the 
shunt,  as  in  a  compound-wound  generator,  opposes  it. 
The  windings  are  so  proportioned  that  the  weakening 
effect  of  the  series  coils  just  compensates  for  the  fall  in 
speed  due  to  increased  load.  By  this  means  is  secured 
a  constant  speed  for  wide  variation  of  load.  The  regula- 
tion is  perfect,  however,  only  when  the  difference  of 
potential  across  the  mains  is  that  for  which  the  motor 
is  designed.  Such  a  motor  must  be  started  in  the  same 
manner  as  a  shunt  motor,  otherwise  the  strong  initial 
series  field  would  overpower  the  shunt  field  and  reverse 
the  direction  of  rotation.  Another  device  which  would 
give  the  motor  the  advantage  of  great  starting  power 
would  be  to  temporarily  reverse  the  series  field  while 
starting,  making  it  agree  with  the  shunt,  and  after  full 
speed  is  attained  and  the  armature  current  falls  to  its 
normal  value,  to  successively  short  circuit  and  reverse 
the  series  coil,  restoring  it  to  its  normal  condition  of 
opposing  the  shunt  winding. 

The  efficiency  of  the  compound-wound  motor  is  lower 
than  that  of  the  compound-wound  generator  or  any  other 
generator,  for  more  energy  is  expended  in  the  field  to 
produce  a  given  magnetization,  some  of  the  lines  of  force 
being  always  neutralized. 

There  are  various  mechanical  devices  for  speed  regula- 
tion, none  of  which,  however,  approach  the  differential 
compound-wound  motor  in  point  of  merit.  We  have 
seen  that  with  increasing  load,  motor  brushes  require 
greater  and  greater  backward  or  negative  lead,  as 


54  TESTING    OF    DYNAMOS    AND    MOTORS. 

opposed  to  the  forward  or  positive  lead  of  generator 
brushes.  This  is  so  because  the  lead  depends  upon  the 
armature  reaction.  If  the  motor  field-polarity  and 
direction  of  rotation  concur  with  that  of  the  dynamo,  the 
direction  of  the  motor  armature  current  is  opposite  to 
that  of  the  dynamo.  This  reverses  the  armature  polar- 
ity, and  with  it  the  field  distortion,  making  the  neutral 
line  recede  instead  of  advance,  and  necessitating  the 
backward  lead.  Hence  the  trailing  pole  corners  are 
strengthened  by  the  distortion,  become  the  seat  of  eddy 
currents,  and  grow  warm.  Again,  in  the  motor  the  arma- 
ture and  whatever  resisting  force  is  attached  to  it  is 
pulled  around  by  the  conductors;  while  in  a  generator 
the  engine  must  turn  the  armature  and  pull  the  conduc- 
tors around  against  the  drag  of  the  magnetic  field. 

Continuous  current  generators  can  in  all  cases  be  used 
as  motors,  though  some  are  better  adapted  than  others. 
Small  motors  cannot  always  be  profitably  used  as  self- 
exciting  generators,  because,  while  in  a  motor  the  field  is 
independent  of  the  armature,  and  the  exciting  energy 
comes  from  without,  and  whatever  amount  may  be 
needed  to  overcome  the  reluctance  of  the  magnetic 
circuit  is  furnished  over  and  above  what  is  needed  for 
the  armature,  in  a  generator  the  exciting  energy  must  be 
supplied  by  the  armature,  and  if  the  reluctance  be  high, 
the  energy  demanded  by  the  field  may  sxceed  the  total 
output  of  the  machine.  In  small  motors  the  air  gap  is 
very  wide  relative  to  the  size  of  the  machine,  so  that  the 
magnetic  reluctance  is  large. 

To  have  the  same  rotation,  the  relation  of  armature 
and  field  polarity  must  in  the  motor  be  the  reverse  of 
that  in  the  generator.  A  series  motor  and  generator  run 


ELEMENTS    OF    THE    MOTOR.  55 

in  opposite  directions  for  given  connections,  and  to  have 
them  turn  in  the  same  direction,  either  the  field  or  arma- 
ture terminals  must  be  reversed  when  the  nature  of  the 
machine  is  reversed.  The  shunt  dynamo  and  motor  run  in 
the  same  direction  for  given  connections,  while  the  direc- 
tion of  rotation  of  a  compound-wound  motor  will  depend 
upon  the  relative  strength  of  series  and  shunt  windings. 
These  points  will  be  explained  elsewhere.  The  ease  with 
which  a  shunt  machine  may  be  reversed  is  illustrated 
where  several  are  in  multiple  on  the  same  bus-bars.  If 
for  any  reason  the  voltage  on  one  machine  falls  far  below 
that  on  the  others,  it  will  not  only  lose  its  own  load,  but 
will  absorb  energy  from  the  others  and  run  as  a  motor, 
and  since  the  rotation  is  the  same  there  will  be  no  violent 
demonstration.  The  brushes  will  spark  some,  the  oppo- 
site side  of  the  belt  will  become  slack,  and  the  ammeter 
needle  will  go  to  zero  and  will  reverse,  or  rise  again, 
according  as  it  is  of  the  Weston  or  Edison  type.  In  any 
case,  whether  the  machine  runs  as  a  motor  or  not  can  be 
ascertained  by  strengthening  its  field  and  watching  the 
ammeter.  If  the  current  rises,  the  machine  is  a  dynamo, 
but  if  it  falls,  a  motor.  In  the  latter  case  the  field  can 
be  gradually  strengthened  till  the  machine  loses  its  load 
as  a  motor,  when  the  ammeter  will  register  zero,  and  if 
the  field  is  further  strengthened  the  needle  will  rise, 
showing  the  machine  to  be  now  acquiring  load  as  a  gen- 
erator. It  is  partly  on  account  of  this  liability  to  rever- 
sal that  shunt  dynamos  have  been  superseded  by 
compound  machines  on  street  railway  service,  where  the 
load  fluctuations  are  very  violent  and  between  wide 
limits. 


THE  TESTING  AND  USE  OF  INSTRUMENTS 


THE  TESTING  AND  USE  OF  INSTRUMENTS. 


CHAPTER   III. 

OHM'S  LAW. 
OHM'S  law  in  its  original  form  is 


where  7  is  the  current  flowing  in  the  conductor,  E  the 
potential  difference  at  the  terminals,  and  R  the  con- 
ductor's resistance.  To  better  comprehend  all  that  this 
law,  in  its  various  forms,  means,  let  us  sec  upon  what 
foundation  it  rests.  In  words  it  can  be  stated  thus:  /// 
any  circuit  containing  only  ohmic  resistance,  the  ralue 
of  the  current  in  amperes  is  expressed  by  the  fraction 
whose  numerator  is  the  fall  of  potential  throughout  the 
circuit,  and  whose  denominator  is  the  resistance  therein. 
In  formulating  such  a  law  we  seek  for  an  expression 
which  shall  give  the  relation  between  the  current,  volt- 
age, and  resistance  of  a  circuit,  and  at  the  time  of  Ohm's 
investigation  it  is  likely  that  he  did  not  know  but  that 
there  were  other  factors  than  E  and  R  to  modify  the 
current  value.  Current  first  attracts  the  attention  of 
the  investigator,  because  it  is  the  current's  actions  that 
are  manifest  to  us:  It  does  the  work,  or  is  the  "vehicle 
of  energy."  When  we  see  a  coil,  around  which  a  current 

59 


60  TESTING    OF    DYNAMOS    AND    MOTORS. 

flows,  suck  up  a  piece  of  iron,  the  presence  of  a  current 
suggests  itself  to  us  rather  than  the  voltage  to  which 
it  is  due,  for  we  can  more  readily  form  an  idea  of 
current  flow  than  of  the  voltage  causing  it.  This  may 
have  influenced  Ohm  in  expressing  his  law  as 

/-  — 
instead  of 

£  =  -j-  or  E  =  I R, 

neither  of  which  latter  forms  conveys  the  same  physical 
meaning  as  the  original.  Having  satisfied  ourselves  as 
to  a  definition  of  current,  we  next  investigate  voltage, 
the  agent  causing  this  current,  and  ascertain  the  quanti- 
tative relation  existing  between  the  two — if  E  is  increased 
what  change  takes  place  in  the  value  of  /?  If  E  is 
decreased,  how  is  /affected?  Ohm  did  not  know  but  that 
doubling  E  might  treble  or  square  /,  until  he  experi- 
mentally proved  that  any  change  in  E  caused  a  like 
change  in  I;  if  E  were  doubled  /  was  doubled;  if  E 
were  halved,  so  was  /,  and  this  fact  is  stated  by  saying 
that  /  varies  directly  as  E. 

The  law  now  is  in  reality  complete  for  a  given  con- 
ductor, for  we  have  found  the  relation  existing  between 
E  and  /.  It  was  early  observed,  however,  that  the  value 
of  /  for  a  given  E  in  conductors  of  similar  dimensions, 
but  of  different  substances,  is  not  the  same;  nor  for 
various  sized  conductors  of  the  same  substance.  This 
modifying  property  which  a  conductor  has  by  virtue  of 
its  form  and  nature,  is  termed  resistance,  and  its  study  has 
been  fruitful  of  valuable  results.  In  experimenting  Ohm 
found  that  by  increasing  the  length  or  decreasing  the 


OHM'S    LAW.  6 1 

cross-section  of  a  conductor,  the  current  was  decreased, 
and  vice  versa.  This  sounds  reasonable,  because  in  in- 
creasing the  length,  the  given  E.  M.  F.  has  a  longer  path 
through  which  to  urge  the  current  of  electricity,  and  in 
decreasing  the  cross-section  the  path  becomes  smaller.  In 
either  case  the  E.  M.  F.  meets  more  obstruction  in  forcing 
the  electricity  through,  and  therefore  cannot  do  so  at  so 
fast  a  rate.  This  is  what  is  meant  by  saying  that  the  con- 
ductivity of  a  wire  decreases  or  the  resistance  increases 
with  the  length,  the  reverse  being  true  for  changes  in 
the  cross-sections.  Now,  were  the  dimensions  all  that 
influence  the  resistance  of  a  conductor,  we  might  formu- 
late a  law  showing  the  relation  between  these  properties, 
and  containing  only  these,  but  it  is  an  established  fact 
that  a  conductor's  resistance  is  modified  by  its  nature  or 
the  substance  of  which  the  conductor  is  composed;  /'.  e.t 
the  dimensions  in  all  cases  being  the  same,  a  silver  wire 
will  have  less  resistance  than  a  copper  one;  an  iron  wire 
more  than  a  copper  one,  the  difference  perhaps  being 
due  to  difference  in  the  molecular  structure  of  the 
bodies. 

To  talk  intelligently  about  this  property,  known  as  spe- 
cific conductivity,  we  must  have  some  standard  to  which  we 
can  refer.  The  standard  generally  accepted  is  pure 
copper.  If  a  pure  copper  wire  of  given  dimensions  has 
a  certain  capacity  for  carrying  current,  and  another  wire 
of  similar  dimensions  but  of  different  substance  has 
twice  the  capacity  of  the  first,  the  specific  conductivity 
of  the  latter  is  2.  This  2  is  fixed  for  the  given  substance, 
being  dependent  solely  upon  the  material  of  the  sub- 
stance, and  hence  it  is  a  constant.  For  the  same  metal 
referred  to  copper  this  constant  is  of  course  always  the 


62  TESTING    OF    DYNAMOS    AND    MOTORS. 

same;  but  for  different  substances,  or  for  the  same 
substance  in  a  different  degree  of  purity,  it  varies, 
and  is  commonly  designated  as  K.  When  K  occurs  in 
a  formula  we  know  that  it  represents  a  number  which  we 
must  either  get  from  tables  compiled  from  the  experi- 
ments of  others,  or  determine  it  ourselves. 

Now  that  we  know  all  the  modifying  factors,  we  must 
determine  the  exact  extent  to  which  resistance  depends 
upon  each.  The  relation  of  R  to  /and  E  is  contained 
in  the  definition  of  resistance  and  in  Ohm's  law,  as 
already  given  in  the  form,  current  value  varies  directly  as 
the  E.  M.  F.;  i.  e.,  when  the  E.  M.  F  increases,  the  cur- 
rent value  increases  at  the  same  rate.  The  relation  sought 
is :  the  current  value  varies  inversely  as  the  resistance  of 
the  conductor ',  i.  e.,  when  the  resistance  increases,  the  cur- 
rent value  decreases  at  the  same  rate.  To  make  this 
plainer,  conductivity  is  defined  as  the  property  of  a  wire 
to  carry  current,  and  resistance  as  the  property  of  check- 
ing back  current  and  preventing  its  rising  to  an  infinite 
value  for  a  finite  E.  M.  F.  Anything  that  increases  the 
resistance,  then,  decreases  the  conductivity,  and  this 
fact  is  expressed  by  saying  that  conductivity  is  the  recip- 
rocal of  resistance;  or,  mathematically, 

Cond.  — 


Res.' 

where  it  can  be  seen  that  increasing  the  resistance 
increases  the  fraction's  denominator,  thereby  decreasing 
the  fraction's  value  and  the  conductivity  to  which  it  is 
equal. 

Let  us  now  suppose  a  current  to  be  flowing  in  a  con- 
ductor of   a   certain   resistance;   let   this   resistance  be 


OHM'S    LAW.  63 

doubled.  What  do  we  mean  by  doubling  R  ?  Since  R 
is  the  power  of  checking  back  /,  and  this  power  is  doubled, 
we  simply  mean  that  R  is  increased  until  /  is  halved. 
When  /falls  to  half  its  first  value  R  is  doubled.  This 
takes  place  when  either  the  length,  Z,  of  the  conductor 
is  doubled  or  its  cross-section,  A,  halved.  So  we  may 
write 

R         A  fC 
~L 

where  K  is  the  specific  conductivity  above  explained. 
Bearing  in  mind  this  definition  of  R,  Ohm  found  that 
in  every  case  the  number  expressing  the  current  strength 
was  equal  to  the  number  obtained  by  dividing  the  "drop  " 
between  two  points  by  the  resistance  between  them,  all 
being  expressed  in  proper  units.  In  practical  units,  it 
is  as  follows:  To  find  the  amperes  flowing  in  a  circuit, 
divide  the  difference  of  potential,  as  indicated  by  a  volt- 
meter placed  between  any  two  points,  by  the  ohms  resist- 
ance between  those  points,  or 


if  E  —  10,  and  R  —  2,  then 

E         10 
/  =    -g  -       —  —  5  amperes. 

To  retain  Ohm's  law  in  this,  its  original  form,  we  think 
first  of  the  current,  /,  as  that  with  which  we  have  to  do 
primarily;  next  of  E.  M.  F.,  E,  or  that  which  causes  the 
current  flow,  and  last  of  resistance,  R,  as  that  which 
hinders  it.  We  remember  that  the  value  of  /  is  ex- 
pressed by  a  fraction,  and  to  increase  the  value  of  a 
fraction  we  increase  the  numerator  or  decrease  the 


64  TESTING    OF    DYNAMOS    AND    MOTORS. 

denominator,  so  that  to  increase  /  we  increase  E  or 
decrease  R,  since  E  is  the  numerator  and  R  the 
denominator.  The  law  of  the  circuit  is  then  expressed 
by  the  fraction. 

Ohm's  law  in  this,  its  simplest  form,  is  found  to  hold 
good  only  on  circuits  where  there  is  no  source  of 
C.  E.  M.  F.  Thus,  if  two  equal  batteries  be  connected 
in  opposition,  /  is  not  equal  to 


but  is  o.  So  also  if  there  is  a  running  motor  in  circuit, 
the  current  value  depends  much  more  upon  the  C.  E. 
M.  F.  of  the  motor  than  upon  its  internal  resistance, 
which  is  made  as  low  as  possible.  Another  example  of 
C.  E.  M.  F.  is  seen  in  the  arc  of  an  arc  lamp.  The  pri- 
mary of  a  transformer,  or  any  device  wherein  self-induc- 
tion takes  place,  is  an  example,  for  self-induction  always 
gives  rise  to  a  C.  E.  M.  F.  In  many  cases  the  self-induc- 
tion lasts  but  a  fraction  of  a  second,  and  /  soon  rises  to 
the  value  called  for  by  Ohm's  law  —  a  shunt  field  winding 
for  example.  Since  self-induction  takes  place  in  a  coil  of 
any  kind,  a  circuit  to  be  practically  free  from  it  must 
consist  of  straight  conductors.  In  some  cases  it  is  nec- 
essary to  neutralize  this  self-induction  by  means  of  an 
equal  and  opposite  induction,  as  in  resistance  boxes  and 
in  the  resistance  coils  of  a  Wheatstone  bridge,  where  the 
wire  is  doubled  in  the  middle  and  then  wound  back  on 
itself.  The  effect  is  to  have  the  current  pass  through 
half  the  coil  right-handed,  and  through  the  other  half 
left-handed,  with  the  result  that  the  opposing  tendencies 
balance  each  other  and  there  is  no  magnetic  effect. 


OHM'S  LAW.  65 

We  lay  stress  on  the  fact  that  Ohm's  law  in  its  simple 
form  holds  good  only  in  non-inductive  circuits.  On  a 
circuit  containing  an  electromagnet,  the  law  applies  only 
after  the  current  has  become  steady.  On  a  circuit  con- 
taining a  running  motor,  the  expression  of  the  law  must 
be  modified.  In  a  circuit  containing  only  ohmic  resist- 
ance, if  we  double  the  voltage  we  double  the  current, 
but  where  there  is  a  running  motor  whose  mechanical 
load  is  kept  constant,  the  effect  of  doubling  the  voltage 
is  to  increase  the  motor's  speed,  its  current  remaining 
almost  the  same.  If  the  voltage  of  the  dynamo  is  E,  and 
the  C.  E.  M.  F.  of  the  motor  is  e,  that  part  of  the 
dynamo  E.  M.  F.  which  drops  through  the  ohmic  resist- 
ance of  this  circuit  is  E  —  c,  or  the  difference  between 
the  impressed  and  C.  E.  M.  F.  But  we  shall  see  later 
that  this  drop  due  to  ohmic  resistance  is  /  R,  where  /  is 
the  current  and  R  the  ohmic  resistance  of  the  circuit; 
these  two  values,  being  identical,  equal  each  other,  and 
we  have  E  —  e  =  I  R,  whence,  as  we  shall  see  presently, 


calling  E  —  t,  E',  we  get 


the  simpler  form.  The  impressed  voltage  equals  the 
drop  due  to  resistance  plus  the  motor's  C.  E.  M  F.,or 
E  —  e  +  I R\  therefore  e  —  E  —  I  R,  or  to  get  the  C.  E. 
M.  F.  of  a  motor,  subtract  the  drop  due  to  internal 
resistance  of  the  motor  from  the  impressed  E.  M.  F., 
as  measured  by  a  voltmeter  at  the  motor  terminals. 
On  a  series  motor  this  internal  drop  =  /  (rf  -f-  ra),  where 


66  TESTING    OF    DYNAMOS    AND    MOTORS. 

rf  and  ra  are  the  resistances  of  field  and  armature 
respectively.  For  a  shunt  machine  the  drop  across  field 
and  armature  circuits  is  the  same,  and  can  be  figured 
from  either  thus,  internal  drop  =  7a  rM  where  7a  is  the 
armature  current.  On  compound-wound  machines  with 
the  long  shunt  connection,  the  drop  is  7a  (ra  -f-  ^s)  where 
rs  is  the  series  field  resistance.  In  the  case  of  the  short 
shunt  (see  Fig.  18)  the  series  field  current  exceeds  the 
armature  current  since  it  includes  that  of  the  shunt 
field.  The  series  field  drop  is  therefore  slightly  increased, 
though  not  appreciably  when  7a  is  large. 

When  the  circuit  contains  an  induction  coil,  the  action 
upon  applying  voltage  is  as  follows:  Upon  closing  the 
circuit  a  small  current  passes  through;  it  throws  out 
lines  of  force  which  cut  neighboring  conductors  and  set 
up  in  them  an  E.  M.  F.  opposed  to  the  impressed  E.  M. 
F.,  and  whose  magnitude  depends  upon  the  rate  of  cur- 
rent variation,  the  number  of  turns  in  the  coil,  and  upon 
the  magnetic  quality  of  the  core.  When  7  reaches  its 
final  value  the  magnetic  field  is  fully  established,  the  lines 
of  force  become  stationary,  the  C.  E.  M.  F.  becomes  o, 
and  7  then,  and  only  then,  equals 

E 


It  can  now  be  more  plainly  seen  that  under  conditions 
where  the  current  strength  varies  through  such  coils,  the 
ohmic  resistance  is  not  the  only  hindrance  to  be  over- 
come, and  that  Ohm's  law  as  first  expressed  will  no 
longer  give  the  true  relation  between  7  and  E.  The 
original  expression  must  then  be  changed  in  problems 
which  involve  alternating  currents,  where  the  greater 


OHM'S  LAW.  67 

part  of  the  effective  resistance  is  due  to  self-induction. 
Calling  r0  the  ohmic  resistance  of  the  circuit,  and  r{  that 
due  to  the  induction  (sometimes  called  spurious  resist- 
ance), then  we  may  write 

J  —  ~  > 

>'o  +   >'i 

but  even  here  it  must  be  remembered  that  r{  has  not 
a  constant  value,  but  varies  from  o  to  a  maximum  for 
every  alternation  of  the  current.  There  may  be  taken, 
however,  a  mean  value  which  shall  represent  its  effective 
value. 

To  revert  to    the  consideration  of  the  formula 


we  here  assume  that  E  and  R  are  given,  and  that  we  wish 
to  find  /.  If  this  form  can  be  retained  in  mind  there  is 
little  difficulty  in  deriving  from  it  the  expression  for  E 
when  /  and  R  are  known,  and  for  R  when  E  and  /are 
known.  /  can  be  written 


for  dividing  a  quantity  by  unity  does  not  alter  its  value. 
The  law  can  then  be  written 

/_  E 
i  "  R' 

which  is  but  another  way  of  writing  the  proportion, 
/  :  i  ;  ;  E  :  R.  Multiplying  means  together  and  ex- 
tremes together,  we  get,  E  x  i  =  /  X  R,  or  E  =  I  R, 
which  gives  E  when  /  and  R  are  known. 

Next,   take  the  form  E  X  i  =  /  X  R\  from  a  rule  in 
arithmetic,  if  we  have  two  products  equal  to  each  other, 


68  TESTING    OF    DYNAMOS    AND    MOTORS. 

one  can  be  taken  as  the  means,  the  other  as  the  extremes  of 
a  proportion,  hence,  R  :  i  ;  ;  E  :  7,  which  can  be  written 

3T>  77 

J\  & 

I  7 

or,  simply, 

_  E 

which  gives  R  when  7  and  E  are  known. 

The  same  result  can  be  gotten  by  a  method  of  common- 
sense  analysis.     We  start  with 


which  is  to  say,  if  we  divide  E  into  R  parts,  each  part 
will  be  equal  to  /;  therefore 

/  =  one  Itth  of  E  =  ~  of  E  =  ^  X  E. 
K  K 

If  one  ^?th  of  E  equals  /,  R  7?ths,  or  all  of  E,  =  R  times 
as  much  as  one  R\.\\,  or  R  times  /;  i.  e.,  E  —  R  7,  or  the 
drop  of  potential  between  any  two  points  of  a  circuit 
equals  the  product  of  the  current  by  the  resistance  in- 
cluded. Again,  from 


we  have  E  =  7  R  as  above,  or  /  times  R  equals  E.  If  / 
times  R  =  E, 

i  E 

R  —  one  7th  of  E  =  j  x    E  =    -j  , 

or  the  resistance  between  any  two  points  of  a  circuit 
equals  the  drop  of  potential  between  those  points  divided 
by  the  current  flowing. 

Resistance  has  been  spoken  of  as  a  sort  of  obstruction 


OHM'S  LAW.  69 

to  the  current,  very  much  as  we  speak  of  friction  in  a 
water  pipe,  or  that  caused  by  the  pebbles  in  a  river  bed. 
There  are  many  points  of  similarity,  and  perhaps  the 
term  electrical  friction  could  be  correctly  used.  We  will 
close  this  discussion  of  Ohm's  law  by  looking  at  it  fron« 
another  standpoint: 

The  fact  or  law  of  the  transformation  of  energy  from 
one  form  into  another  has  already  been  spoken  of.  It  is 
also  true,  so  far  as  can  be  determined,  that  a  material 
agent  is  always  active  in  such  transformations.  It  has 
been  observed  that  when  current  flows  in  a  conductor  a 
certain  amount  of  heat  is  developed.  This  shows  that  a 
certain  amount  of  electrical  energy  is  being  transformed 
into  the  energy  of  heat.  Careful  experiment  shows  that 
the  amount  of  electrical  energy  so  transformed  equals 
72  /^,  where  /  is  the  current  flowing,  and  R  the  resist- 
ance of  the  portion  of  the  circuit  investigated.  This  law 
is  due  to  Joule,  and  is  called  by  his  name.  To  illustrate 
the  law,  if  /  =  12  amperes,  and  ft  —  10  ohms,  the 
energy  lost  in  heat  =  12*  x  10  —  1,440  watts.  (We 
say  energy  lost  because  ordinarily  it  is  unavailable  for 
any  useful  purpose.)  If  R  be  doubled  the  energy  lost  = 
i2a  x  20  =  2,880  watts.  If  R  be  halved  the  energy  lost 
=  i2a  x  5  =  7 20- watts,  and  so  on.  The  energy  trans- 
formed is,  then,  directly  proportional  to  R,  and  R  may  be 
called  the  conductor 's  property,  by  virtue  of  which  it  can  trans- 
form electrical  energy  into  heat  energy.  This  property  would 
naturally  have  different  values  in  different  substances, 
i.  e.,  each  substance  has  its  own  specific  resistance,  etc., 
and  resistance  may  be  regarded  as  a  measure  of  one  of 
the  fundamental  properties  of  conductors. 


CHAPTER  IV. 

MEASUREMENT    OF    CURRENT. 

IN  studying  the  factors  involved  in  an  electric  circuit, 
no  better  order  can  be  selected  than  that  in  which  they 
occur  in  Ohm's  law.  We  shall  then  take  up  in  succession, 
Current  Strength,  Electromotive  Force,  and  Resistance, 
and  consider  in  a  common-sense  way  the  methods  em- 
ployed in  their  measurement  along  the  most  practical 
lines  of  industrial  work. 

To  do  quantitative  work  of  any  kind,  there  must  be 
chosen  proper  units  in  terms  of  which  operations  and 
results  can  be  expressed.  The  universally  adopted  units 
of  current  strength,  E.  M.  F.,  and  resistance  are  the 
ampere,  volt,  and  ohm,  which  have  already  been  briefly 
defined.  The  practical  basis  upon  which  the  unit  of  cur- 
rent strength  is  founded  is  the  amount  of  metal  which 
the  current  will  deposit  from  a  solution  of  the  metal  per 
unit  of  time.  When  a  current  is  passed  through  a  solution 
of  a  salt  of  any  metal,  the  salt  is  decomposed  and  the 
metal  deposited  upon  the  negative  plate  or  cathode.  The 
amount  deposited  depends  upon  the  metal,  the  current 
strength,  and  in  lesser  measure  upon  the  nature  of  the 
solution.  That  current  which  deposits  from  a  solution 
of  silver  nitrate  .001118  grams  of  metallic  silver,  or  from 
a  solution  of  copper  sulphate  .000329  grams  metallic 

70 


MEASUREMENT    OF    CURRENT.  7 1 

copper,  per  second,  is  of  unit  strength,  and  is  called  the 
ampere. 

The  deposition  of  metals  is  one  of  the  most  popular 
and  most  reliable  methods  of  measuring  current  strength, 
and  is  used  a  great  deal  in  calibrating  and  standardizing 
the  higher  grades  of  galvanometers,  ammeters,  and 
shunts.  In  every-day  work  it  is  customary  to  employ 
the  method  of  copper  deposition,  as  it  requires  the  use  of 
neither  expensive  apparatus  nor  of  chemically  pure  silver, 
which  is  hard  to  get. 

The  apparatus  required  for  the  copper  method  consists 
of  a  pair  of  scales,  a  timepiece,  copper 
plates  fitted  to  a  wooden  support  which 
holds  them  in  place  in  a  bath  of  copper 
sulphate,  and  the  means  for  cleaning  and 
drying  the  plates.  Fig.  23  shows  a  very 
simple  and  cheaply  made  voltameter.  V  is 


a  glass  vessel    containing   the    solution   of 
copper    sulphate    (blue    vitriol).       T  is   a 
wooden  cover  provided  with  holes  to  receive  the  wires 
connected  with  the  plates  P.     The  plates  are  drawn  up- 
snug  into  slots  to  prevent  turning,  and  are  joined  to  the 
circuit  by  the  connectors  C,  which  are  of  the   ordinary 
type.     It  is  usual   to   make  the  anode  heavier  than  the 
cathode,  for  deposition  is  carried  on  at  its  expense. 

The  chemical  action  in  the  voltameter  is  simple.  The 
current  entering  at  the  anode  causes  the  copper  of  this 
plate  to  pass  into  solution,  and  a  progressive  interchange 
of  copper  in  the  molecules  of  the  solution  between  the 
two  plates  presumably  takes  place,  and  metallic  copper 
is  deposited  on  the  cathode  where  the  current  leaves  the 
voltameter.  It  would  naturally  be  expected  that  the 


72  TESTING    OF    DYNAMOS    AND    MOTORS. 

gain  of  the  one  would  exactly  equal  the  loss  of  the  other, 
and  this  is  true,  excepting  that  the  formation  of  by-prod- 
ucts in  the  solution  makes  the  anode's  loss  exceed  the 
cathode's  gain,  and  it  is  for  this  reason  that  the  cathode's 
gain  in  weight  is  alone  used  in  estimating  current  value. 

In  preparing  the  voltameter  for  use,  the  copper  plates 
(called  electrodes)  must  first  be  cleaned,  dried,  and 
weighed.  In  cleaning,  all  dirt  and  oxides,  shown  by 
discoloration,  are  removed  with  a  scratch  brush;  the 
plates  are  then  put  through  a  series  of  baths,  whose  order 
and  composition  are  variously  given  by  different  writers. 
The  following  commendable  course  is  given  by  Stewart 
and  Gee  in  their  Laboratory  Manual  : 

Bath  No.  i.  Alkaline  liquid  for  cleansing  copper:  i 
part,  by  weight,  of  caustic  soda;  10  parts,  by  weight, 
of  water. 

Bath  No.  2.  Acid  solution  :  i  part,  by  volume,  strong 
sulphuric  acid;  10  parts,  by  volume,  water. 

Bath  No.  3.  Dipping  liquid :  equal  volumes  commer- 
cial nitric  acid  (or  that  left  from  battery  fluid)  and  water. 

Bath  No.  4.  Brightening  liquid :  100  parts,  by  volume, 
strong  nitric  acid ;  i  part,  by  volume,  strong  hydro- 
chloric acid. 

Enough  of  these  solutions  should  be  prepared  to  com- 
pletely cover  the  copper  plates.  No.  i  being  placed  in 
a  porcelain  evaporating  dish,  the  others  in  glass  beakers. 
The  plate  is  first  washed  under  the  tap,  rubbing  the 
plate  well  with  a  rag.  It  is  then  boiled  in  (alkaline) 
liquid  No.  i.  This  will  cause  discoloration,  due  to 
oxidation.  From  this  point  on,  the  plate  should  not  be 
touched  with  the  fingers.  After  boiling  until  the  discol- 
oration is  pronounced,  raise  the  plate  with  a  lifter  and 


MEASUREMENT    OF    CURRENT.  73 

wash  under  the  tap;  then  remove  it  and  place  it  in  liquid 
No.  2  long  enough  to  enable  the  acid  to  dissolve  the  dark 
colored  oxide.  Wash  again  with  water  (distilled  water 
preferred),  and  place  in  liquid  No.  3  for  about  fifteen 
seconds,  after  which  it  is  washed  again  and  dipped  for 
a  few  seconds  in  liquid  No.  4.  As  this  liquid  is  very 
strong,  the  plate  should  be  washed  quickly  and  thor- 
oughly in  distilled  water.  Should  it  now  fail  to  present 
a  bright  clean  surface,  the  process  must  be  repeated. 
Once  bright,  the  plates  are  kept  in  a  dilute  solution  of 
copper  sulphate  until  desired  for  use. 

The  solution  in  the  depositing  bath  is  made  by  dis- 
solving 100  grams  of  copper  sulphate  in  500  cubic  centi- 
metres of  water  (or  17^  ounces  in  one  pint).  Before 
weighing,  the  plates  are  thoroughly  dried  either  by 
placing  them  in  a  vessel  containing  chloride  of  lime, 
which  in  a  short  time  will  absorb  all  moisture,  or  by 
heating,  care  being  taken  that  the  plates  are  not 
touched  by  the  fingers  or  otherwise  soiled.  To  prevent 
rusting,  and  thereby  to  save  much  work  on  future 
occasions,  the  plates,  when  not  in  use,  are  kept  in  a  box 
containing  a  dish  of  chloride  of  lime. 

Before  proceeding  with  a  measurement  we  will  build 
up  the  formula  to  be  used.  We  have  defined  the  ampere 
as  that  current  which  will  deposit  .000329  gram  of 
copper  per  second;  as  it  is  impracticable  to  deal  with 
a  current  that  flows  for  but  i  second,  it  is  allowed  to 
flow  for  an  hour  or  more,  when  the  deposition  which 
takes  place  in  i  second  is  gotten  by  dividing  the  total 
deposition  by  the  number  of  seconds.  This  value  of 
deposition,  made  by  the  unknown  current  in  i  second 
divided  by  .000329,  gives  the  value  of  the  unknown  cur- 


74  TESTING    OF    DYNAMOS    AND    MOTORS. 

rent  expressed  in  amperes.  If  /  is  the  current  to  be 
measured,  K,  the  electro-chemical  equivalent  of  copper 
(/.  e.,  the  .000329  gram  deposited  by  i  ampere  in  i 
second),  and  W,  the  weight  of  the  copper  deposited  by 
current  /  in  /  seconds,  then  W  —  K  1  1  —  weight  depos- 
ited by  i  ampere  in  i  second  X  the  number  of  amperes,  /, 
X  the  number  of  seconds,  /.  From  W  =  Kit,  we  get, 


or  the  unknown  current  equals  the  total  weight  deposited 
divided  by  the  weight  which  i  ampere  will  deposit  in  the 
allotted  time. 

To  conduct  a  test  the  voltameter  is  connected  up  so 
that  the  current  enters  at  the  heavier  plate,  —  anode,— 
and  leaves  at  the  lighter  one,  or  cathode.  A  current 
indicator  and  a  variable  resistance  are  placed  in  series 
with  the  bath.  The  object  of  the  test  is  to  measure  the 
current  flowing  through  the  voltameter,  with  a  view, 
generally,  of  standardizing  some  ammeter  or  galvanom- 
eter. The  current  indicator  need  not  be  calibrated, 
but  by  means  of  the  variable  resistance  the  needle  is 
kept  at  one  spot  throughout  the  test.  Preliminary  to  the 
test  it  is  well  to  use  a  pair  of  little  test  plates  to  insure 
that  deposition  takes  place  in  the  right  direction,  also  it  is 
well  to  secure  approximately  the  current  to  be  used,  with 
the  voltameter  short-circuited,  and  thereby  avoid  the 
error  incident  to  adjustment.  All  being  ready,  the  switch 
is  closed  and  the  time  carefully  noted.  It  is  not  well  for 
the  current  to  exceed  a  density  of  .08  ampere  per  square 
centimetre  (.5  ampere  for  square  inch),  as  the  deposi- 
tion then  goes  on  too  rapidly,  and  the  copper  granu- 


MEASUREMENT    OF    CURRENT. 


75 


lates.  and  falls  to  the  bottom  as  a  sort  of  "mud,"  or 
black  powder. 

The    form    of   voltameter    just  'described    is    capable 
of    giving    results    with   an    error    of    from    i  £  to    4  #, 
according    to     the     experimenter's    skill.     When     more 
accurate    work    is    desired,    the    spiral    coil    form    may 
be   used.       This    has    the    advantage    that   it    is    easier 
to    manipulate,    and     has    a 
normal    error   of   but  0.3  %. 
The  following  description  is 
taken  from   a  paper  read  by 
Professor  H.   J.  Ryan  before 
the    American    Institute    of 
Electrical    Engineers,    May, 
1889:  Two  copper  wires  are 
selected      whose     size     and 
length     are    determined   by 
the  current  to   be  measured. 
They   are    first    cleaned    by 
fastening  one  end  in  a  vise,  FIG.  24. 

and   carefully  sand-papering 

the  surface.  The  wire  is  then  coiled  on  a  cylinder,  care 
being  taken  that  it  does  not  touch  the  hands  at  any  time. 
The  cathode  is  made  into  a  smaller  coil  than  the  anode, 
hence  is  lighter.  Fig.  24  shows  the  final  appearance 
when  mounted.  On  block  A,  a  glass  vessel  £,  is  placed; 
T and  7",  are  two  binding  posts  serving  as  terminals  and 
also  holding  supports,  ct,  <-,,  to  which  are  attached  the 
two  coils  hanging  one  inside  the  other,  but  not  touching. 
Before  use,  the  coils  are  chemically  cleaned  as  fol- 
lows: After  polishing,  the  gain  coil  is  washed  by  plung- 
ing it  into  a  jar  of  water  containing  a  little  sulphuric  acid. 


76  TESTING    OF    DYNAMOS    AND    MOTORS. 

It  is  then  rolled  on  filter  or  blotting  paper  to  remove  all 
but  a  film  of  water.  The  coil  is  then  dipped  in  95  % 
alcohol,  removed,  and  the  excess  of  alcohol  allowed 
to  drip  back  into  the  jar.  By  again  rolling  the  coil  on 
clean  filter  or  blotting  paper,  nothing  but  a  mere  film 
of  alcohol  remains,  and  that  is  thoroughly  evaporated 
in  a  few  moments,  leaving  the  coil  entirely  dry.  Coils 
that  have  become  corroded  can  be  rapidly  cleaned  by 
plunging  into  a  mixture  of  100  parts  strong  nitric  acid, 
i  part  hydrochloric  acid,  and  then  proceeding  as 
already  directed.  At  the  end  of  the  deposit  the  gain 
coils  are  immediately  removed  and  plunged  first  into 
clean  water,  then  into  the  acidulated  water,  from  which 
they  are  dried  by  means  of  the  alcohol.  When  dry  they 
are  ready  to  be  weighed.  The  copper  sulphate,  water, 
and  acid  need  not  necessarily  be  chemically  pure.  The 
density  of  the  voltameter  solution  should  not  be  less  than 
1. 1,  nor  more  than  1. 18,  referred  to  water.  A  voltameter 
of  the  above  type  suitable  for  measuring  currents  up  to 
4  amperes  has  a  gain  coil  of  No.  16  B.  &  S.  wire,  2  1/2 
metres  (8.2  feet)  long.  For  heavier  currents  two  or 
more  voltameters  can  be  placed  in  parallel. 

The  voltameter  cannot  be  used  to  measure  alternat- 
ing currents,  for  any  tendency  to  deposit  on  one  plate  is 
met  at  the  next  alternation  by  a  counter  tendency  to 
deposit  on  the  other.  Nor  is  the  method  applicable  to 
industrial  work  where  the  current  value  must  be  read  at 
a  glance.  It  is,  however,  a  most  valuable  method  in  a 
laboratory  or  testing  room,  for  calibrating  instruments 
which  in  turn  are  to  be  used  as  standards. 

To  standardize  an  ammeter  it  is  connected  in  series 
with  the  voltameter,  as  shown  in  Fig.  25,  where  B  is  the 


MEASUREMENT    OF    CURRENT.  77 

source  of  current,  G  the  meter  to  be  standardized,  R  a 
variable  resistance,  K  a  key  for  closing  the  circuit,  and 
A'    a   key    for    short-circuiting   the    voltameter    V.      In 
making  the  test,  V  is   first   short- 
circuited   and    R  adjusted    till    the 
reading  on  G  is  a  little  above   the 
desired     value,    and     then     A'l    is 
opened.     This   precaution    reduces 
the  error  due  to  a  variation  in  the 
current.      The    test   plates,    above 
referred  to,  can  be  profitably  used,  Fi<;.  25. 

and  may  indicate  the  current  to  be 

flowing  the  wrong  way  ;  if  necessary,  the  battery  leads 
can  be  reversed,  and  this  may  necessitate  reversing  G's 
leads  to  right  its  deflection.  Everything  adjusted,  A\ 
is  opened,  A'  closed,  and  the  time  noted. 

If  the  instrument  under  calibration  is  one  with  a  needle 
and  scale,  such  as  the  Weston  type,  several  points  on  the 
scale  can  be  determined  and  the  intermediate  values 
interpolated,  or  a  curve  can  be  plotted  giving  true  current 
values  and  their  corresponding  scale  readings.  If  G  is  a 
tangent  galvanometer,  it  is  usual  to  determine  what 
current  causes  a  deflection  of  45°,  and  the  current 
value  corresponding  to  any  other  deflection  can  be 
figured  from  the  formula  /  =  K  tan  a,  where  a  is  the 
deflection,  /the  current  to  be  found,  and  A' that  value 
of  /  giving  a  45°  deflection.  The  formula  /  =  A"  tan 
a  is  the  equation  of  the  tangent  galvanometer,  and  in 
words  may  be  stated  thus:  The  current  passing  through 
a  tangent  galvanometer  is  equal  to  the  tangent  of  the 
angle  of  deflection  multiplied  by  a  constant.  The  angle, 
a,  is  read  on  the  instrument  itself,  its  tangent  is  gotten 


78  TESTING    OF    DYNAMOS    AND    MOTORS. 

from  a  table  of  tangents.  The  constant,  K,  which  has 
been  defined,  is  called  the  constant  of  the  galvanometer, 
and  is  different  for  different  instruments.  The  factors 
upon  which  it  depends  are  the  number  of  turns  of  wire  in 
the  coils,  the  coil's  average  diameter,  and  the  horizontal 
component  of  the  earth's  magnetic  field.  Why  this  is  true 
can  in  a  measure  be  gathered  from  the  following  brief 
outline  of  galvanometer  principle  placed  here  for  those 
readers  to  whom  it  may  not  be  familiar.  Every  magnet  has 
lines  of  force,  or  a  field,  and  any  magnetic  needle  placed 
near  the  magnet  will  take  up  a  position  from  which  it  will 
resist  being  turned.  The  magnetic  field,  then,  has  a 
directing  force  over  the  needle,  and  holds  it  in  a  position 
to  give  the  longest  metal  path  to  the  lines  of  force. 
Now,  the  earth  is  a  huge  magnet,  having  lines  of  force 
with  definite  direction,  and  these  lines  exert  a  directing 
or  guiding  force  on  all  magnetic  materials.  If  a  needle 
be  suspended  or  pivoted  in  air,  it  takes  up  a  position 
depending  upon  the  direction  of  the  earth's  lines  at  that 
place;  and  on  galvanometers  having  only  the  earth's  field 
as  a  directing  force,  the  frame  of  the  instrument  must  be 
turned  until  the  earth's  magnetic  influence  holds  the 
needle  over  the  zero  mark.  If  a  magnet  be  brought 
near  the  needle,  it  will  leave  its  zero  position  and  take 
up  a  new  on.e  where  the  directing  force  and  opposing 
deflecting  force  balance  each  other.  If  the  magnet  be 
removed,  the  needle  ought  to  resume  its  original  posi- 
tion. Now  replace  the  magnet  by  an  electromagnet, 
i.  ^.,  wind  around  the  needle  turns  of  wire  capable  of 
carrying  a  current,  and  we  have  a  galvanometer.  Next 
send  a  current  through  the  coils  and  the  needle  deflects; 
increase  the  current  and  the  deflection  increases. 


MEASUREMENT    OF    CURRENT.  79 

Weaken  the  directing  force  by  opposing  a  magnet  to  the 
earth's  field,  and  the  deflection  increases.  Decrease  the 
diameter  of  the  coils,  bringing  the  current  nearer  to 
the  needle,  and  the  deflection  becomes  greater;  increase 
this  diameter  and  the  reverse  is  true.  Anything  which 
increases  the  directing  force  or  decreases  the  deflecting 
force,  lessens  the  deflection,  and,  rife  versa,  to  decrease 
the  directing  force  or  increase  the  deflecting  force 
increases  the  deflection. 

The  earth's  lines  of  force  do  not  run  parallel  to  the 
surface  except  at  the  equator,  while  at  the  poles  they 
run  perpendicular  to  the  surface.  At  places  between  the 
poles  and  equator  the  lines  run  at  an  angle  to  the  earth's 
surface,  and  therefore  have  two  effects  upon  a  suspended 
needle  :  one  is  a  vertical  pull  which  tries  to  make  the 
needle  dip  ;  the  other,  which  is  the  one  we  have 
most  to  do  with,  acts  upon  the  needle  horizontally,  and 
is  known  as  the  horizontal  component  of  the  earth's 
magnetism. 

Since  the  earth's  field  has  a  slight  cyclic  and,  indeed, 
daily  variation,  A",  in  the  strictest  sense,  does  not  remain 
constant;  but  as  it  is  influenced  vastly  more  by  the 
proximity  of  masses  of  iron  or  steel,  it  may  be  regarded 
as  constant  so  far  as  the  earth  is  concerned.  The  value 
of  K  is  found  as  follows:  in  the  above  vqltametric  test 
adjust  the  needle  to  a  deflection  of  45°,  and  by  means  of 
the  voltameter  determine  what  current  causes  this  deflec- 
tion. The  tangent  of  45°  =  i,  hence  the  above  expres- 
sion becomes,  /  =  K  x  i  =  K ;  but  /  is  the  current 
which  gives  a  deflection  of  45° ;  therefore  K  has  the  same 
value  as  /.  For  any  other  deflection  I  and  K  have 
different  values.  While  this  form  of  the  experiment  is 


80  TESTING    OF    DYNAMOS    AND    MOTORS. 

simplest  and  best,  because  the  needle  is  most  sensitive  in 
this  part  of  the  scale,  and  tangent  45°  =  i,  any  other 
angle  can  be  used  in  determining  K. 

A  serious  deviation  from  the  above  law  indicates  either 
an  imperfection  in  the  instrument,  the  presence  of  some 
outside  influence,  or  lack  of  adjustment  of  the  coils  to  the 
magnetic  meridian,  /.  e.  f  direction  in  which  the  needle 
points  when  influenced  only  by  the  earth.  This  latter 
adjustment  is  secured  by  moving  the  instrument  until  the 
same  deflection  on  opposite  sides  of  o  is  gotten  upon 
reversing  the  galvanometer  current.  If  the  galvanom- 
eter is  of  the  reflecting  type,  /.  e.t  with  mirror  and 
telescope,  or  lamp,  and  scale,  it  is  not  feasible  to  obtain 
a  deflection  of  45°,  so  that  the  deflection  is  not  read  in 
degrees  at  all,  but  in  terms  of  the  scale  divisions.  The 
value  of  one  scale  division  depends  upon  the  distance  of 
the  scale  from  the  mirror.  The  farther  off  the  scale,  the 
greater  the  number  of  divisions  through  which  a  given 
deflecting  force  will  cause  the  needle  to  throw  the  ray  of 
light.  This  distance,  then,  should  be  fixed  once  for  all. 
Another  factor  of  great  importance  in  determining  the 
value  of  a  division  is  the  damping  or  directing  magnet, 
which  is  free  to  turn  or  slide  up  and  down  on  a  rod  above 
the  instrument.  The  directing  force  of  this  magnet  can 
be  made  so  strong  as  to  render  the  needle  independent  of 
outside  disturbing  fields. 

If  these  points  remain  fixed,  their  absolute  distance  need 
not  be  known,  it  being  customary  to  determine  what  cur- 
rent causes  a  deflection  of  100  or  200  scale  divisions.  With 
the  scale  at  least  3  feet  from  the  mirror,  we  can  assume, 
without  sensible  error,  that  the  current  is  proportional  to 
the  deflection  as  read  on  the  scale,  so  that  with  the  cur- 


MEASUREMENT    OF    CURRENT. 


8l 


rent  value  of  one  point  determined,  all  others  can  be 
figured  out.  Thus,  suppose  the  current  for  100  divisions 
is  .05  =  5/100  ampere,  what  is  the  current  for  45 
divisions?  The  proportion  is  .05  :  x  j  |  100  ;  45, 
whence  jc  =  .0225  ampere.  Or,  if  100  divisions  be  due 
to  .05  ampere,  i  division  is  due  to  i/ioo  of  5/100 
ampere  =  5/10,000,  and  45  divisions  would  be  due  to  45 
times  this,  or  225/10,000  =  .0225  ampere. 

Most  reflecting  galvanometers  are  extremely  sensitive, 
and  but  a  suggestion  of  current  will  throw  the  ray  off  the 
scale.  By  shunting  the  instru- 
ment, however,  it  may  be  used 
to  measure  currents  many 
thousands  of  times  its  own  ca- 
pacity. For  practical  meas- 
urements of  current  it  is  usual 
to  set  up  these  instruments  to 
read  from  a  reliable  standard 
shunt  of  known  resistance. 

The  theory  of  the  shunt  is  briefly  as  follows:  Let  rt  rt 
be  the  resistances  of  the  two  branches  included  between 
A  and  B  of  Fig.  26.  The  fall  of  potential  from  A  to  B  is 
the  same  along  either  branch.  Let /be  the  total  current 
and  /',  and  /,  the  currents  in  rl  and  r3,  respectively. 
/  =  t\  -f-  /,  and,  from  Ohm's  law,  the  drop  from 
A  to  B  —  t\  rv  But  it  also  =  /,  rs.  Therefore, 
/,  rt  =  /a  ra ,  and  we  have  an  equality  of  two  products, 
whence  from  arithmetic  we  get,  t\  :  /a  '  ;  ;-a  :  rt.  That  is 
to  say,  the  currents  in  the  two  branches  are  to  each 
other,  inversely,  as  the  resistances  of  these  branches. 
For  example,  let  rg  be  the  resistance  of  the  branch, 
including  the  galvanometer,  G,  Fig.  27,  and  rB  that  of 


FK;.  26. 


82 


TESTING    OF    DYNAMOS    AND    MOTORS. 


shunt  box,  S,  then,  t'B  '•  tg  '.'  rg  :  ra,  whence  taking  prod- 
uct of  means  and  extremes  and  solving  for  zg,  we  get 


If 


and 


r  g  an  rs  are  known,  and  fg  is  measured,  /8  is  also 
determined,  and  since  /  =  ig  +  /s,  the  current  in  the 
external  circuit  is  known.  Since 


to  get  z'g,  we  need  not  know  the  absolute  value  of  either 
r9  or  rgt  but  only  their  ratio,  or  the  value  of  the  fraction 


Furthermore,  it   is  convenient  to   have   this  fraction  a 
simple  one,  such  a  i/io,  i/ioo,  i/iooo.     This  makes  the 
shunt  current  10,  100,  or  1,000  times  as  great  as  that  read 
on  the  galvanometer,  and  the  total  current  10  -f  i,  100  -f- 
i,  or  1,000  -f-  i  times  the    galvanometer  current.     Now 
this,  while  vastly  better  than  any  haphazard  value,  can 
be   improved  upon.     Thus,  instead  of  i/io,  i/ioo,  etc., 
put  1/9,  1/99,  J/999-     The  shunt 
current  is    now,    9,    99,    and  999 
times    (according    to  shunt  used) 
as  great  as  the  galvanometer  cur- 
rent,    while    the    total,    or    line, 
current  is  9  -f  i  =  10  times,  99  -f- 
i  =  TOO  times,  or  999  -f-  i  =  1,000 
FIG.  27.  times,   the  galvanometer  current. 

With    this  arrangement  the  value 

read  on  the  scale  is  i/io,  i/ioo,   or  1/1,000  of  the  total 
current,  /,  and   G  becomes  direct  reading. 


MEASUREMENT    OF    CURRENT.  83 

In  accordance  with  this  theory,  the  finer  instruments 
are  provided  with  a  shunt  box,  which  has  resistances 
bearing  the  ratios  1/9,  1/99  and  1/999  to  that  of  the 
galvanometer.  Each  box  can  be  used  only  with  its  own 
galvanometer,  or  one  similar  in  all  respects,  and,  in  very 
fine  work,  only  at  temperatures  approximating  that  at 
which  it  was  standardized. 

In  the  calibration  of  a  sensitive  galvanometer,  intended 
for  current  measurements,  the  constant  is  determined  by 
use  of  shunt  and  voltameter.    It 
can  be  then  used  on  any  direct 
current  circuit,    and   with   any 
shunt   box   whose  resistance   is 
known.     The    connections    for 
the  test  are  shown  in  Fig.   28, 
where   G   is  a  galvanometer  of  Fli;   2g 

known  or  easily  determined  re- 
sistance, S,  the  shunt,  also  of  known  resistance,  in  multi- 
ple with  G\  V  is  the  voltameter,  A',  its  cut-out  switch, 
B  a  battery  or  other  current  source  whose  voltage  can 
be  varied  within  wide  limits;  R  is  a  variable  resistance 
for  further  adjustment,  r  and  r  resistances  in  series 
with  G.  On  high  resistance  galvanometers  intended  for 
measuring  current,  voltage,  and  resistance,  r,  r  is  gen- 
erally divided  into  two  boxes,  one  of  which  has  a  constant 
value  of  75,000  or  100,000  ohms,  the  other  a  variable  one 
of  10,000  to  15,000. 

Suppose  our  scale  has  200  divisions,  and  it  is  desired  to 
calibrate  it  throughout.  Call  G's  resistance  1,000  ohms: 
and  r's  105,000,  making  a  total  of  106,000  ohms  in  G's 
circuit.  Let  the  shunt  resistance,  S,  be.i  ohm  (i/io). 
Now  close  A'and  K'  and  adjust  R  until  £'s  needle  deflect? 


84  TESTING    OF    DYNAMOS    AND    MOTORS. 

200.  To  avoid  exceeding  the  current  carrying  capacity 
of  S,  it  should  be  protected  by  a  fuse  or  short-circuiting 
switch;  and  it  is  well  to  have  in  circuit  a  current  indicator 
as  a  guide  to  preliminary  adjustment.  If  it  is  impossible 
to  get  a  deflection  of  200  by  means  of  -/?,  r  may  be 
decreased  or  the  resistance  of  the  shunt  increased.  Cir- 
cumstances dictate  the  position  of  the  directing  magnet, 
and  where  the  instrument  is  set  up  amid  disturbing 
external  influences,  it  is  not  well  to  move  it  far  from 
the  needle.  The  proper  deflection  secured,  Kv  is  opened, 
time  noted,  R  quickly  readjusted,  if  necessary,  and  the 
deflection  of  200  maintained  for  about  an  hour.  The 
current  value  is  determined  by  the  method  of  weighing 
as  already  explained.  Suppose  this,  /,  to  have  been  20 
amperes.  The  problem  now  is  to  find  the  value  of  G's 
current.  We  have  as  data,  /  =  20  amperes;  resistance 
of  galvanometer  circuit  —  106,000  ohms;  resistance  of 
S  =  .  i  ohm.  From  the  theory  of  shunts  already  out- 
lined, we  get  the  proportion,  ie  :  4  *  *  .  i  :  106,000,  whence 

.  i  i 


8  io6,ooo         s  1,060,000 

also,  ig  +  is  =  I  —  20  amperes,  or  ia  =  20  —  fe 
Substituting  this  value  of  4  in  the  expression, 

i 

Iff       ^^       la        7 J 

I,O6O,OOO 

we  have 

.  . .  i_      _   20  —  fg 

*g  ~  '  **'i,o6o,ooo    "  1,060,000' 

Multiplying  the  quantities  on  both  sides  of  the  equality 
sign  by  1,060,000  and  we  get,  1,060,000  ie  —  20  —  i& 


MEASUREMENT    OF    CURRENT.  85 

or  1,060,001  /    =  20,  and  /',,=    •  -   amperes 

1,060,001 

through  the  galvanometer  circuit.  Expressed  decimally, 
this  is  .0000189  ampere,  or  that  current  which,  passing 
through  G,  under  existing  circumstances,  causes  a  deflec- 
tion of  200  scale  divisions,  and  is  commonly  accepted  as 
the  instrumental  constant  of  the  galvanometer.  If  there 
are  no  variable  external  influences  (such  as  heavy  ma- 
chines under  test,  passing  cars  or  trains,  traveling  cranes, 
elevators,  etc.),  and  the  instrument  is  carefully  set  up 
with  scale  at  a  proper  distance,  we  are  safe  in  assuming 
that  if  .0000189  ampere  deflects  the  needle  200  divisions, 
1/200  part  of  it  will  cause  a  deflection  of  i  division. 
This  would  be  .000000094  ampere,  and  more  properly 
constitutes  the  constant  of  the  instrument.  To  elimi- 
nate errors  of  adjustment  and  observation  it  is  well  to 
experimentally  fix  at  least  three  points  on  each  side  of  o. 

In  setting  up  a  galvanometer  calibrated  elsewhere,  and 
with  the  same  shunt,  S,  it  is  unnecessary  to  repeat  the 
entire  calibration.  Before  removal,  a  portable  instru- 
ment, such  as  a  Weston  milliammeter,  can  be  placed  in 
circuit  and  the  indication  corresponding  to  several  points 
on  the  galvanometer  scale  marked  on  the  scale  of  the 
Weston  instrument.  Then  upon  resetting  the  galvan- 
ometer the  current  can  be  adjusted  by  this  indicator, 
and  the  deflection  on  £'s  scale  adjusted  to  the  proper 
value  by  the  controlling  magnet. 

We  found  .000000094  ampere  in  the  galvanometer  cir- 
cuit to  cause  a  deflection,  of  i  division,  and  this  we  called 
later  the  instrumental  constant.  With  the  .  i  ohm  shunt  a 
deflection  of  200  corresponded  to  a  current  of  20  amperes 
in  the  outside  circuit;  this  makes  a  deflection  of  i  division 


86  TESTING    OF    DYNAMOS    AND    MOTORS. 

correspond  to  an  external  current  of  .1  ampere,  and  this 
we  call  the  working  constant.  For  example,  a  deflection 
of  47  indicates  a  current  of  47X-i  =  4.7  amperes. 

From  the  instrumental  constant  can  be  determined  the 
working  constant  of  any  other  shunt,  without  using  the 
voltameter.  Suppose  we  use  a  .25  ohm  shunt:  with 
the  same  current  as  before,  the  ray  is  thrown  quite  off 
the  scale,  and  the  deflection  must  be  reduced  to  be  read- 
able. This  done,  let  us  inquire  what  line  current  a  de- 
flection of  200  indicates.  From  the  theory  of  the  shunt 
given  above,  we  have, 

galvanometer  current  :  shunt  current  ',  ',  shunt  resist- 
ance :  galvanometer  resistance,  or,  ig  :  is  '  ;  .25  :  -106,000, 
whence 

^t?     —  —     ^8     X     '         "2  ~     - 

IO6,OOO 


For  a  deflection  of  200  we  have  seen  that  t'g  =  .0000189 
ampere,  and  substituting  this  in  the  expression  for  /g, 
we  get, 

.0000189  =  -  !  —  , 
424,000 

or  /3  =  .0000189  X  424,000  =  8.0136  amperes,  the  shunt 
current;  this  added  to  tg  =  .0000189  ampere,  gives  the 
line  current.  Practically  speaking,  the  following  propor- 
tion holds  good,  20  :  8  ;  ;  .25  :  .i.  Showing  that  when 
the  resistance  in  the  galvanometer  circuit  is  so  great  that 
the  galvanometer  current  can  be  neglected,  we  may  say 
that  for  the  same  deflection,  with  two  different  shunts, 
the  line  currents  are  inversely  as  the  resistance  of  the 
shunts.  Thus  with  a  shunt  of  .03  ohm  a  deflection  of 
200  indicates  what  current?  Here  we  have  20  :  x  \\ 
.03  :  .1,  or  .03.*  =  2,  whence  x  =  66.66  amperes. 


MEASUREMENT    OF    CURRENT.  87 

More  exactly  it  is  66.69,  which  is  approximately  the  same. 
With  such  a  galvanometer  and  its  circuit  resistance 
undisturbed,  it  is  a  simple  matter  to  use  any  shunt  of 
known  resistance. 

Another  way  of  altering  the  range  of  readings,  is  to 
leave  S  the  same,  and  vary  the  galvanometer  circuit 
resistance.  Let  us  suppose  that  this  resistance  is  to  be 
reduced.  Since  more  current  will  now  flow  in  G,  for  a 
given  line  current,  the  deflection  will  be  greater.  The 
following  is  a  simple  way  to  proceed:  Adjust  the  line 
current  so  that  the  deflection  shall  be,  say,  50  divisions, 
showing  the  line  current  to  be  5  amperes.  Then  cut  out 
resistance  from  G's  circuit  till  the  deflection  is  100.  A 
deflection  of  100  now  corresponds  to  5,  and  200  to  10, 
amperes.  This  assumes  the  line  current  to  have  been 
kept  constant  throughout.  We  can,  however,  verify 
results  by  checking  up.  Since  the  galvanometer  current 
can  be  neglected, the  drop  through  the  shunt  is  .  i  x  5  —  .5 
volts.  If  we  are  to  produce  a  deflection  of  100,  the  galvan- 
ometer current  required  for  this,  is,  .000000094  x  100  = 
.0000094  ampere,  .000000094  ampere  being  the  current 
to  deflect  the  ray  i  division.  Since  a  potential  difference 
of  .5  volt  is  applied  to  £'8  circuit,  we  have  by  Ohm's  law, 

.0000094  =  :-  , 

where  x  is  the  galvanometer  circuit  resistance  under  the 
new  conditions.  Solving  for  .r,  we  find  x  =  53, 191  ohms, 
and  the  first  resistance,  106,000  ohms,  must  be  reduced  to 
this  value.  If  experiment  and  theory  fail  to  check  up  very 
closely,  it  indicates  either  an  error  in  the  determination  of 
the  galvanometer  constant,  or  inaccuracy  in  the  resistance 


88 


TESTING    OF    DYNAMOS    AND    MOTORS. 


boxes.  Here  again  we  see  the  same  rough  proportionality 
as  before  :  halving  the  galvanometer  circuit  resistance  Jias 
doubled  the  deflection  due  to  a  given  current.  A  third 
way  of  increasing  the  reading  range  is  to  draw  out  the 
scale  till  its  o  occupies  the  TOO  or  200  point.  On  a 
properly  adjusted  instrument  with  correct  constant,  this 
introduces  no  error. 

Thus  far  we  have  assumed   the  controlling  magnet  to 
occupy  a  constant  position,  a  condition  hard  to  secure. 

Fig.  29  gives  connections  for 
a  method  of  setting  up  a 
galvanometer  independent  of 
all  local  conditions,  and  one 
popular  in  actual  practice.  G 
is  a  galvanometer  with  a  di- 
recting  magnet.  7?  is  a  vari- 
able, and  £'  a  constant  resist- 
ance. B  is  a  standard  cell  whose  voltage  is  accurately 
known,  and  K  a  key.  Resistance,  r,  completes  the 
battery  circuit,  and  the  galvanometer  reads  the  drop  of 
potential  off  a  known  fraction  of  r.  This  resistance,  r, 
must  be  so  high  as  to  produce  no  perceptible  effect  upon 
the  cell's  terminal  voltage  when  the  circuit  is  closed. 
Let  r  be  10,000  ohms,  and  the  E.  M.  F.  of  the  cell,  a 
Daniell,  i  volt.  If  the  resistance  of  the  galvanometer 
circuit  is  high,  it  can  be  placed  as  a  shunt  across  any 
part  of  the  box  without  affecting  the  resistance  of  the 
battery  circuit.  The  drop  of  potential  through  any  por- 
tion of  the  box  is  proportional  to  the  resistance  of  this 
portion;  this  enables  us  to  apply  to  the  galvanometer  cir- 
cuit any  fraction  of  the  cell's  E.  M.  F.,  by  simply  taking 
the  drop  from  that  same  fraction  of  r.  Thus  let  the 


MEASUREMENT    OF    CURRENT.  89 

resistance  included  be  1,000  ohms  (—  i/io  of  10,000),  or 
i/io  of  the  box;  then  the  potential  difference  applied  to 
£'s  circuit  is  i/io  volt.  Should  we  do  away  with  a  high 
resistance  galvanometer,  and  with  ft  and  ft',  and  suppose 
the  galvanometer  circuit  resistance  to  be  but  rooohms,  it 
will,  when  shunted  across  any  part  of  /-,  perceptibly  lower 
the  resistance  between  the  points  included,  and  the  pre- 
vious condition  no  longer  exists :  the  current  is  increased, 
and  the  potential  difference  at  the  galvanometer  terminals 
no  longer  bears  the  same  ratio  to  the  total  voltage  of  the 
cell  that  the  included  resistance  of  the  box  does  to  the 
total  resistance.  With  a  high  resistance  galvanometer  cir- 
cuit,of  10,000  ohms  and  upward, the  effect  upon  the  circuit 
can  be  neglected.  In  the  connections  considered  above, 
the  galvanometer  resistance  was  1,000  ohms,  and  ft  and  ft' 
respectively  5,000  and  4,000  ohms,  making  the  total  gal- 
vanometer circuit  resistance  10, ooo  ohms.  With  a  current 
of  .0000188  ampere  (corresponding  to  a  deflection  of  200), 
the  potential  difference  through  the  circuit  is  by  Ohm's 
law,  E  —  I  R,  =  .0000188  x  10,000  =  .188  volt.  This 
may  be  called  the  working  voltage  constant.  The 
instrumental  voltage  constant  is  .000000094  x  1,000  = 
.000094  volt.  That  is  to  say,  if  .000094  volt  be  applied 
to  the  galvanometer  terminals,  a  deflection  of  one  division 
will  result.  We  now  place  the  galvanometer  circuit  across 
2/10  of  the  box,  and  have  .2  volt  applied  to  this  circuit. 
This  should  produce  a  deflection  of  213  divisions.  For  if 
200  divisions  are  due  to.i88  volt,  i  division  is  due  to 
1/200  of  .188  volt,  or  .188/200  =  .00094  volt,  and  if 
.00094  volt  causes  a  deflection  of  i,  .2  volt  will  cause  a 
deflection  =  .2  -j-  .00094  =  212.7,  practically  213  divi- 
sions. Having  .2  volt  on  the  galvanometer  circuit,  it  only 


90  TESTING    OF    DYNAMOS    AND    MOTORS. 

remains  to  adjust,  by  means  of  the  controlling  magnet,  the 
deflection  to  213.  This  must  be  done  by  raising  or  lower- 
ering  the  magnet,  and  not  by  rotating  it.  To  test  the 
adjustment,  reverse  the  current  through  the  galvanom- 
eter circuit:  the  deflections  on  opposite  sides  of  o  should 
be  equal. 

The  galvanometer  adjusted,  any  standard  shunt  can  be 
used,  and  the  scale  indications  calculated  as  in  the  last 
case.  For  example,  with  a  .01  ohm  shunt  we  have  the 
proportion  .01  :  10,000  ;  ;  .0000188  :  x,  where  x  is  the 
current  in  the  shunt.  Whence,  by  arithmetic,  x  =  18.8 
amperes.  A  deflection  of  200,  then,  corresponds  to  a 
current  of  18.8  amperes,  100  divisions  to  9.4  amperes. 

If  nothing  is  known  regarding  the  galvanometer  save 
its  resistance,  the  method  just  described  can  be  used  as 
follows,  to  set  it  up  and  calibrate  it,  and  without  recourse 
to  a  voltameter.  The  galvanometer  circuit  is  placed 
across  a  part  of  the  box  r,  say  i/io  of  it,  and  the  con- 
trolling magnet  adjusted  to  give  a  certain  deflection,  say 
100.  With  the  resistance  of  the  galvanometer  circuit 
known,  its  current  is  found  by  Ohm's  law,  and  from  this 
current  both  the  voltage  and  current  constant  are  deter- 
mined. If  it  is  desired  not  to  disturb  the  magnet,  the 
resistance  of  R'  can  be  varied  until  the  deflection 
sought  is  gotten.  This  is  allowable  only  so  long  as  the 
resistance  of  the  galvanometer  circuit  is  not  reduced  too 
much,  and  to  avoid  this  condition  it  is  customary  to 
Hiave  R  so  high  as  to  avoid  error,  even  though  R'  be  all 
cut  out. 

The  range  for  which  a  galvanometer  is  adjusted  to 
read  current  should  not  be  so  great  as  to  make  the 
lower  readings  inaccurate.  As  a  guard  against  this,  the 


MEASUREMENT    OF    CURRENT.  91 

galvanometer  must  be  so  adjusted  that  the  maximum  cur- 
rent in  the  shunt  corresponds  to  the  highest  possible 
deflection.  If  under  this  condition  the  lower  readings 
are  still  liable  to  error  of  observation,  it  is  best  to  divide 
the  readings  into  two  parts — either  using  two  shunts  cr 
two  adjustments  of  the  galvanometer.  In  setting  up  the 
instrument  it  is  desirable  to  know  the  current  range  as 
well  as  resistance  of  at  least  one  shunt,  so  that  some 
estimate  can  be  made  of  the  potential  difference  to  which 
the  galvanometer  is  to  be  subjected.  Suppose  shunt  No. 
i  to  have  a  resistance  of  .001  ohm,  and  a  carrying  capac- 
ity of  100  amperes:  100  amperes  x  .001  ohm  —  i  volt, 
which  is  to  be  impressed  upon  the  galvanometer  circuit 
at  full  load.  Fora  minimum  load  of  i  ampere,  the  poten- 
tial difference  impressed  will  be  i  x  .001  .=  .001  volt. 
The  conditions  must  be  such  that .  i  volt  will  give  a  deflec- 
tion of  say  300  scale  divisions.  This  can  be  secured  by 
shifting  the  magnet  and  varying  the  resistance  in  A'1, 
while  the  galvanometer  includes  1,000  ohms  in  r.  One 
ampere  will  now  give  a  deflection  of  3  divisions,  or  i/ioo 
of  what  100  amperes  give.  Having  made  the  scale  direct- 
reading  for  one  shunt,  the  readings  for  the  other  shunt 
are  a  matter  of  calculation.  Thus,  if  the  resistance  of 
No.  2  be  half  that  of  No.  i,  and  of  twice  its  current 
capacity,  it  will  require  the  full  200  amperes  to  give  the 
full  deflection  of  300  divisions.  This  gives  us  i  1/2 
divisions  per  ampere,  and  this  shunt  can  be  used  for 
currents  between  100  and  200  amperes  with  the  same 
degree  of  accuracy  as  No.  i  can  for  those  between  i  and 
100  amperes. 

Standard    shunts  are   furnished   with    a   guarantee    of 
their  resistance  at  a  given  temperature,  and  tables  show- 


92  TESTING    OF    DYNAMOS    AND    MOTORS. 

ing  their  resistance  at  other  temperatures  can  be  had. 
In  ordering,  the  desired  current  carrying  capacity  should 
be  specified.  On  the  other  hand,  it  is  not  difficult  to 
make  a  shunt  that  will  serve  all  ordinary  purposes.  The 
precision  of  the  work  will  depend  largely  upon  the 
sensitiveness  of  the  galvanometer,  for  if  it  is  not  very 
sensitive,  minute  variations  in  resistances  will  not  affect 
the  deflection.  Let  the  range  of  the  proposed  shunt  be 

1  to  100  amperes,  and  let  the  galvanometer  scale  contain 
200  divisions.     We  first  determine  what  voltage  must  be 
used  to  cause  this  deflection  of  200.     Assume  it  to  be  .  i 
volt.     The  resistance  of  the  shunt   must  then    be   .001 
ohm,  for  (Ohm's  law) 

J?  =  —.  =  .1  -+-  100  =  .001. 

The  next  step  is  to  select  conductors  of  proper  resistance 
and  of  capacity  to  carry  the  current  without  undue  heat- 
ing. The  wire  table  (see  appendix)  shows  No.  10  B.  & 
S.  copper  wire  to  carry  40  amperes  with  little  heating,  and 
to  measure  i  ohm  per  1,000  feet.  One  foot  then  measures 
.001  ohm;  five  wires  in  multiple,  and  each  i  foot  long, 
measure  1/5  of  .001  ohm  =  .0002  ohm,  and  will  carry  200 
amperes.  Five  wires  in  multiple,  and  5  feet  long,  measure 
.0002  x  5  =  .001  ohm,  and  will  also  carry  200  amperes. 
This  then,  furnishes  the  desired  combination.  Allowing 

2  inches  for  the  soldering,  the  wires   are  cut    5    feet    2 
inches  long,  and  are  drawn  into  holes  bored  into  copper 
lugs,  shown  at  A  B  in  Fig.  30.     The  mortised  joints  are 
then  well  sweated  with  solder,  and  the  whole  mounted  on 
a  wooden  base.     On  the    lugs  are  two  sets   of  binding 
posts;  one  set  to  serve  as  the  main  circuit  connection, 
the  other  for  the  terminals  of  the  galvanometer  circuit, 


MEASUREMENT    OF    CURRENT.  93 

and  care   must   be  taken    that    these    terminals   do    not 
make  a  bad  joint  in  the  main  circuit,  for  this  would  be 
equivalent   to    increasing 
the     shunt's     resistance. 
In    testing  the    shunt  by 
the  standard  cell  and  re- 
sistance box,  there  should  I- K..  30. 
be     provided     a      3-way 

switch,  by  which  the  galvanometer  can  be  shifted  alter- 
nately to  the  standard  cell  and  to  the  shunt.  Through- 
out this  test,  as  in  all  others,  readings  should  be  taken 
on  both  sides  of  o. 

In  ammeter  calibration  it  is  usual  to  have  one  man 
look  after  the  galvanometer,  while  an  assistant  varies  the 
current  strength  as  required  and  marks  the  deflection  of 
the  needle  if  the  scale  is  ungraduated,  or  reads  out  the 
deflection  on  a  graduated  scale.  The  galvanometer  man 
makes  a  record,  in  parallel  columns,  of  the  galvanometer 
and  corresponding  ammeter  readings,  and  these  can  be 
plotted  as  a  curve  for  future  reference.  If  the  variation 
exceeds  20  or  25  %  the  instrument  should  be  returned 
to  the  maker  for  correction. 

In  graduating  new  ammeters  they  are  all  joined  in  series. 
The  current  is  adjusted  to  the  proper  value  by  the  galva- 
nometer, and  this  is  marked  under  the  needle  of  the  meter ; 
the  next  highest  current  is  then  secured  and  the  marking 
repeated,  and  so  on  to  the  limit  of  the  scale.  If  of  differ- 
ent current  capacities,  switches  must  be  provided  to  cut 
out  each  instrument  when  its  limit  is  reached.  Ammeters 
of  very  different  capacity  may  have  scale  cards  of  the 
same  size,  the  divisions  of  the  higher  reading  meter  being 
close  t<  gather  and  more  in  number.  If  a  i5o-ampere  card 


94  TESTING    OF    DYNAMOS    AND    MOTORS. 

have  every  tenth  graduation  elongated  and  numbered, 
it  can  be  used  on  a  i5-amperemeter  without  further 
change,  the  smaller  divisions  now  indicating  tenths  of  an 
ampere.  On  meters  used  for  large  currents  it  is  custo- 
mary for  the  makers  to  furnish  a  shunt  to  be  used  with 
it.  The  ammeter  terminals  go  to  the  shunt  which  is  in 
the  main  circuit,  and  the  card  is  scaled  off  as  before. 
Although  but  a  small  current  goes  through  the  meter  its 
needle  indicates  the  main  circuit  current.  In  the  finer 
Weston  meters  the  shunt  is  contained  within,  but  on  the 
station  instruments  the  shunt  is  a  separate  device,  the 
meter  itself  appearing  more  like  a  voltmeter.  An 
observer  may  be  puzzled  by  the  presence  alone  of  two 
small  flexible  cords  on  an  instrument  registering  perhaps 
2,000  or  3,000  amperes,  the  shunt  in  most  cases  not  being 
in  sight.  Where  commercial  ammeters  are  turned  out 
in  quantities,  the  galvanometer  is  replaced  by  a  standard- 
ized meter  which  is  recalibrated  at  regular  intervals  by 
the  galvanometer  and  shunt.  By  means  of  this  second- 
ary standard,  calibrating  can  be  done  rapidly.  Readings 
above  i  ampere  are  taken  in  steps  ascending  and  de- 
scending, the  two  sets  of  readings  differing  more  or  less 
according  as  iron  does  or  does  not  enter  into  the  working 
part  of  the  instrument.  The  descending  readings  will 
be  higher  than  the  ascending  ones  for  the  same  current, 
on  account  of  the  residual  magnetization  of  the  iron.  The 
descending  readings  are  influenced  not  only  by  the  cur- 
rent, but  by  the  fact  that  the  iron  is  more  strongly 
magnetized  at  the  time  they  are  taken.  If  the  current  is 
kept  constant  for  some  time,  and  the  instrument  tapped 
with  the  finger,  the  needle  will  return  to  the  ascending 
deflection  for  that  current  value.  The  mechanical  fric- 


MEASUREMENT    OF    CURRENT.  95 

tion  of  moving  parts  also  has  its  influence  upon  the 
ascending  and  descending  readings,  and  in  the  rougher 
forms  of  instrument  the  needle  often  fails  to  return  to 
o  after  the  circuit  is  opened. 

in  instruments  depending  for  action  upon  springs, 
permanent  magnets,  electromagnets  without  iron  cores, 
and  the  heating  and  consequent  expansion  of  wires, 
errors  due  to  residual  magnetism  are  absent.  In  the 
well-known  Weston  instruments,  the  directing  force  is  a 
permanent  magnet,  two  delicate  springs  the  restraining 
force,  and  a  coil,  free  to  turn  on  jeweled  bearings,  the 
deflecting  force.  The  permanent  magnet  produces  a 
strong  magnetic  field  and  protects  the  needle  from 
external  disturbing  fields.  When  handled  with  care,  they 
should  show  no  variation  even  after  months  of  use. 

The  Ayrton  and  Perry  instruments  are  examples  of 
the  electromagnetic  type  whose  deflecting  and  directing 
forces  both  depend  upon  the  working  current.  The 
Cardew  instruments  depend  upon  the  elongation  of  a 
wire  when  heated  by  a  current.  As  the  wire  expands  it 
actuates  a  roller  to  which  is  attached  the  index. 

Ammeters  of  any  sort,  like  galvanometers,  may  have 
their  capacity  increased  by  means  of  a  shunt.  The 
resistance  of  this  shunt  depends  upon  the  multiplying 
power  it  is  desired  to  give  the  meter,  and  is  measured  by 
the  quotient 


where  R8  is  the  shunt  resistance,  Rg  that  of  the  galvan- 
ometer, and  n  the  desired  multiplying  power,  or  the  num- 
ber of  times  the  meter's  capacity  is  to  be  multiplied.  That 


96 


TESTING    OF    DYNAMOS    AND    MOTORS. 


this  is  so  can  be  seen  from  the  following:  We  have 
learned  elsewhere  that  the  ratio  of  the  currents  in  the 
branches  of  a  divided  circuit  is  the  inverse  ratio  of  the 
resistance  of  those  branches.  If  in  Fig.  31  i\  *2,  indi- 

cate the  currents,  rt  J\  the 
resistances  of  the  branches, 


then  t\  :  *a  :: 
may  write  it, 


or  we 


FIG.  31 


Let    the    main   current  be 
40  amperes,  and  suppose  we 

wish  30  amperes  to  go  through  the  shunt  and  10  through 

the  meter;  then, 

*',          10  /,          rn 


therefore, 


30 


r  i\j  i      '  i  'Sv-'  i 

-  =  ~  and  ^  :     ^  =  3,  and  r,  =  3  r,  , 

where  r}  =  galvanometer  resistance,  and  r^  that  of  the 
shunt.  If  10  amperes  is  the  meter  or  galvanometer  read- 
ing, when  40  amperes  are  in  the  line,  the  multiplier  is  4. 
Taking  the  formula  above, 


n  —  i 


and  substituting  for  RK  its  value  3  R^  we  get 


dividing  through  by  ^8  we  have 


n  —  i 
whence,  n  —  i  =  3  and  n  =  4,  as  above. 


MEASUREMENT    OF    CURRENT.  97 

Take  now  a  general  case.  What  must  ^8  be  to  have 
i/;/  of  the  line  current  go  through  the  meter  G?  For  this 
purpose  the  combined  conductivity  of  galvanometer  and 
shunt  must  be  known.  To  get  a  wire's  conductivity,  we 
divide  i  by  the  wire's  resistance,  and  in  this  case 

-'-   and  ~ 
*g          Xt 

are  the  respective  conductivities  of  the  galvanometer  or 
meter,  and  of  the  shunt,  and 


their  combined  conductivity.  The  line  current  is  the 
sum  of  the  branch  currents,  and  the  current  in  any 
branch  divided  by  the  total  or  line  current  equals  the  con- 
ductivity  of  the  branch  divided  by  the  total  conductivity, 
hence,  if  /t  is  the  line  current,  and  i/n  of  this  goes 
through  the  ammeter,  the  following  proportion  is  true: 


/  •  /  •  •  -L  •  * 

Jt  ••<«..  -p     •   — 


or 


1*      _L 

/t  :       n 


whence,    R^  -f-  R%  =  n   R9\    subtracting  Jt,  from   both 
sides,  we  have,  R9  =  n  Jt^  —  Kn  or  R9  =  Rt  (n  —  i)  and 


as  above.     If  n  is  to  equal   10,  then  ^B  =  1/9  Itv  etc 
We  must  distinguish  between  the  condition  that  a  certain 


98  TESTING    OF    DYNAMOS    AND    MOTORS. 

fraction  of  the  whole  current  shall  pass  through  the 
ammeter,  and  the  condition  that  the  shunt  current  shall 
be  a  certain  number  of  times  greater  than  that  in  the 
meter;  in  the  above  case  where  n  —  10,  the  main  current 
is  10  times,  and  the  shunt  current  9  times,  that  in  the 
meter. 

The  effect  of  shunting  an  instrument  is  to  decrease  the 
total  resistance  of  the  circuit  and  thereby  increase  the 
total  current.  In  ammeter  work  this  effect  is  negligible, 
but  in  galvanometer  work  a  compensating  resistance  is 
put  in  circuit.  If  the  galvanometer  resistance  is  R%,  and 
that  of  the  shunt  R^  their  multiple  resistance  is 


and  RZ  — 


is  the  resistance  of  the  compensating  coil.  This  coil  is 
used  only  in  very  delicate  work,  for  ordinarily  the 
resistance  in  circuit  is  so  high  that  any  small  variation 
can  be  neglected. 

To  shunt  an  ammeter,  and  increase  its  capacity  by  any 
desired  amount,  is  a  very  simple  operation,  and  can  be 
done  without  previous  calculation.  Suppose  we  have  a 
loo-ampere  ammeter,  and  we  wish  to  use  it  for  reading  as 
high  as  500  amperes.  The  meter  must  carry  one-fifth 
and  the  shunt  four-fifths  the  total  current.  The  wire  to 
be  used  in  shunting  must  carry  400  amperes  without 
heating.  In  Fig.  32,  A  and  B  are  the  terminals  to  the 
meter,  A,  to  which  the  shunt,  S,  is  to  be  attached.  One 
end  of  6"  is  fixed  at  A,  and  the  other  end  is  left  free  to 
slide  through  a  clamp  at  B.  Before  connecting  6*  to  B, 
adjust  the  current  at  100  amperes,  then  insert  S,  and 
draw  it  through  until  the  reading  falls  to  20.  If  this  is 


MEASUREMENT    OF    CURRENT.  99 

done   quickly  we   can   assume   that  the  current  has  not 

varied   during  the  operation.     Otherwise  we   must  place 

between   S  and    //   a 

switch,  which   can   be 

repeatedly  opened,  to 

insure  that  the  deflec- 

tion   is      always     100 

when   the   whole   cur- 

rent     flows     through 

A  ',  and  that   it    is  20  when  S  is  in  circuit.     To  get  the 

line  current    we    have  now  only  to    multiply  the  meter 

current  by  5.     The  strong  point  in  favor  of   this  most 

practical    method  is   that    no  resistance  values  need  be 

known. 

If  circumstances  call  for  a  predetermination  of  the 
shunt  resistance,  that  of  A  must  be  known.  Knowing 
this,  and  the  proposed  relation  of  S's  current  to  A"s,  S 
can  be  figured.  Suppose  that  with  a  2oo-amperemeter, 
whose  resistance  is  .002  ohm,  we  are  to  measure  a  current 
of  500  amperes.  S  must,  then,  carry  300  amperes,  and 
the  relation  of  A's  current  to  S's,  is 

7A>   _    200   _    2 

™          ~  3  ' 


The  ratio  of  the  respective  resistances  must  then,  be 
3  :  2  or 


whence  the  proportion,  7A':  /„  ;  *  Rn  :  R^  and  substi- 
tuting the  known  value,  we  have,  2  :  3  •  '  R%  :  .002,  or 
£<,  =  .00133  ohm.  All  resistance  must  be  measured 


100  TESTING    OF    DYNAMOS    AND    MOTORS. 

from  post  to  post,  so  that  the  joints  may  be  common  to 
both  paths,  otherwise  an  error  may  be  introduced. 

In  place  of  a  shunt,  a  second  ammeter  may  be  used 
in  multiple  with  the  first.  In  such  a  case,  although  the 
carrying  capacity  of  the  meters  in  multiple  may  exceed 
the  current  to  be  read,  it  does  not  follow  that  each  will 
take  its  proper  share;  one  needle  may  swing  off  its  scale 
A  and  the  other  read 

low.  In  order  that 
the  meters  may  read 
proportional  parts  of 
the  total  current, 
their  resistances 
must  be  inversely 
proportional  to  their 
current  capacities. 
For  example  in  Fig.  33  let  A  and  Al  be  two  meters 
of  100  and  300  amperes  capacity  respectively,  and 
the  current  to  be  read  400  amperes.  If  A's  resistance  is 
three  times  ^4/s,  each  will  read  its  full  capacity,  and  the 
sum  will  be  400.  If  A's  resistance  is  twice  A^s,  A  will 
take  133  amperes,  throwing  its  needle  off  the  scale,  while 
Al  will  take  but  266  amperes,  two-thirds  of  its  capacity. 
To  rectify  matters  a  resistance  R  must  be  put  in  series 
with  Al  and  of  such  value  as  to  establish  the  proper  rela- 
tion between  the  two  readings.  This  can  be  done  either 
experimentally  or  by  calculation.  If  A  measures  .004 
ohm,  and  Al  .002  ohm,  the  resistance  necessary  to  put  in 
with  A  is  .002  ohm,  for  since  A's  capacity  is  but  one-third 
A^s,  its  resistance  must  be  three  times  A^'s  to  have  the 
current  divide  proportionately.  If  A's  is  .004,  and  we 
add  R  =  .002,  we  get  .006  (=  3  X  .002),  which  is  what 


MEASUREMENT    OF    CURRENT.  IOI 

we    wish.      By  proportion   we    have,    100  :  300   ;  ;   .002  : 
(.v  -|-    .004)    where    x  is    the    value   of    A'.       Multiplying 
together  the  means  and  the  extremes  we  get 

(x  -p-  .004)  100  =  300  x  .002. 
100  x  -f-  -4  =  -6,  and  .v  =  .002  ohm. 

If  the  adjustment  is  made  experimentally  with  the  meters 
in  circuit,  R  is  varied  till  ,7's  reading  is  three  times  that 
on  A^  and  this  ratio  should  hold  on  all  parts  of  the  scale. 
Laws  demonstrated  for  two  wires  or  meters  in  multiple 
hold  for  any  number  in  multiple.  Suppose  it  is  desired  to 
replace  one  wire  carrying  10  amperes  by  three  wires  which 
shall  carry  7,  2,  and  i  amperes  respectively,  what  must  be 
their  resistances?  Calling  the  conductivity  of  the  com- 
bined wires  10,  that  of  the  separate  wires  must  be  7/10, 
2/10,  and  i/io  respectively.  Their  resistances  must  be 
in  the  same  ratio  as  the  reciprocals  of  the  conductivities, 
or  as  10/7-:  10/2  :  10/1,  which  is  1.4  :  5  :  10.  No  par- 
ticular values  can  be  given  as  answers  to  the  problem 
thus  stated,  since  any  three  resistances  which  are  to  each 
other  as  1.4.:  5  :  10,  will  cause  the  current  to  divide  in 
the  proper  ratio,  but  if  we  modify  the  proposition  by 
assuming  the  resistance  of  the  original  wire  to  be  known, 
the  resistance  of  each  branch  can  be  determined.  Thus, 
if  we  assume  the  resistance  of  the  single  wire  to  be  2 
ohms,  then  with  a  current  of  10  amperes,  the  potential 
difference  across  their  common  junction  will  be  (/  x  R  = 
10  x  2)  20  volts.  Since  the  fall  of  potential  along  each 
of  the  branches  is  to  be  20  volts,  the  resistance  of  each 
will  be  20  divided  by  the  respective  currents.  Thus  the 
7-ampere  branch  must  have  a  resistance  of  20/7  ohms,  = 
2.86  ohms;  the  2-ampere  branch,  20/2  =  10  ohms,  and 
the  last  20/1  =  20  ohms. 


102  TESTING    OF    DYNAMOS    AND    MOTORS. 

The  same  problem  may  arise  in  a  different  way.  Sup- 
pose we  are  given  three  conductors  of  2,  3,  and  4  ohms  re- 
spectively; when  connected  in  multiple  on  the  line,  how 
will  the  current  divide  among  them?  Suppose  the  line 
current  to  be  10  amperes,  with  the  resistances  2,  3,  and  4, 
the  conductivities  will  be  1/2,  1/3,  and  1/4  respectively, 
and  their  combined  conductivity  1/2  -|-  1/3  -f-  1/4  = 
13/12;  the  multiple  resistance,  being  the  reciprocal  of 
this,  is  12/13  °f  an  ohm.  With  a  current  of  10  amperes 
the  potential  difference  across  their  junction  is  12/13  X 
10  =  9.23  volts.  The  respective  currents  will  now  be, 

ii  =  2^3  _  4  6j-  amperes;  /2  —  9Ji3  —  3.07, 

and  /    =  ?-'— 5  ==  2.31  amperes, 
4 

and  their  sum  9.99  amperes,  very  nearly  10  amperes,  the 
current  conditioned.  This  is  as  convenient  a  method  as 
any  for  solving  problems  of  this  character,  though  gen- 
eral results  may  be  derived  from  abstract  theory  and 
formulae  obtained,  in  which  we  need  only  to  substitute  the 
values  given.  Thus  for  the  multiple  resistance  of  three 
wires  the  formula  is, 

R  =  - 


*    +     'I    'I    +    ',    ^3 

where  rl9  r^  and  r3  are  the  respective  resistances  of  the 
three  wires.     Substituting  above,  we  get, 

R  =  - *  X  3  X  4 =  ££  as  above< 

2X3  +  3X4  +  2X4        13 

Circumstances  sometimes  arise  where  the  only  reliable 


MEASURING    OF    CURRENT. 


103 


ammeter  must  be  sent  away  for  repairs,  and  some  tempo- 
rary expedient  must  he  resorted  to  for  replacing  it.  The 
most  practicable  plan  is  to  use  a  voltmeter  in  connection 
with  a  known  resistance.  In  anticipation  of  this,  a  piece 
of  copper  conductor  should  be  placed  in  circuit  with  the 
ammeter  and  the  drop  across  it  taken  for  different  cur- 
rent values.  Where  the  voltmeter  is  too  high  reading, 
its  calibrating  coil  can  be  used.  The  method  is  not  to  be 
recommended  for  extensive  testing  rooms,  where  the 
range  of  current  to  be  read  is  large,  nor  is  it  capable  of 
great  accuracy,  but  for  a  single  machine  it  suffices  to 
keep  the  current  constant 
during  a  heat  test,  and  is 
especially  well  adapted  to 
testing  series  machines, 
where  the  series  coil  can  be 
used  as  the'standard  resist- 
ance.  Fig.  34  shows  the 
connections  for  using  a 
single  voltmeter  to  read 
both  current  and  voltage  in  a  street  railway  motor  test. 
V  is  the  voltmeter,  C,  one  terminal  of  the  calibrating  coil, 
its  other  terminal  being  in  common  with  the  high  voltage 
terminal  at  S,  T  is  the  trolley,  G  the  ground,  F  the 
motor  field,  A  the  armature,  and  R  the  resistance  by 
means  of  which  we  are  to  read  the  current.  When  the 
full  line  connection  is  made,  the  high  voltage  coil  is  in 
action  and  the  instrument  reads  the  voltage  of  the  line. 
With  the  dotted  connection,  the  calibrating  coil  gives  the 
drop  on  R.  Care  must  be  taken  in  making  connections 
that  the  calibrating  coil  is  not  subjected  to  the  full  line 
voltage. 


Fir..  34. 


104 


TESTING    OF    DYNAMOS    AND    MOTORS. 


The  methods  of  current  measurement  so  far  given  have 
been  those  by  copper  deposition,  by  the  shunted  galvanom- 
eter, the  ammeter,  and  the  voltmeter,  indirect,  but  none 
of  these  have  a  universal  application.  Deposition  cannot 
be  used  at  all  for  alternating  currents,  while  galvanometers 

and  meters  must  have 
special  features  to  fit 
them  for  this  work. 
Instruments  depend- 
ing upon  the  heating 
and  consequent  ex- 
pansion of  wires  may 
be  used;  also  those 
depending  upon  the 
attraction  of  a  solen- 
oid upon  a  piece  of 
soft  iron  free  to  move 
in  a  magnetic  field. 
The  Cardew  instru- 
ments are  types  of  the 
former,  the  Edison, 
Westinghouse  and 
Thomson-Houston  of 
the  latter. 

The  ideal  instrument  for  use  in  alternating  work  is 
the  Siemens  dynamometer.  The  following  description 
is  adapted  from  Ayrton's  ''Practical  Electricity."  The 
instrument  belongs  to  the  class  of>  spring  control  meters 
and  its  principle  is  essentially  that  of  the  electric  motor, 
one  coil  corresponding  to  the  field,  the  other  to  the 
armature,  the  two  coils  being  generally  connected  in 
series  as  are  the  fields  and  armature  of  a  series  motor. 
The  effect  of  reversing  either  or  both  of  the  coils  is  the 


FIG.  35. 


MEASUREMENT    OF    CURRENT. 


same  as  that  seen  in  a  motor  under  similar  treatment. 
The  dynamometer  is  shown  in  perspective  in  Fig.  35  and 
symbolically  in  Fig.  36.  It  consists  of  a  fixed  coil 
A  B  C  D  and  a  movable  coil  E  F  G,  the  latter  being 
frequently  made  of  a  single  stiff  wire.  Connection  is 
made  to  the  movable  coil  by  two  mercury  cups  into 
which  its  terminals  dip.  The  suspension  is  by  means  of 
a  silk  thread  and  a 
delicate  spiral  spring 
resists  the  turning 
force  due  to  the  cur- 
rent. A  pointer  M 
shows  the  angle 
through  which  the 
spring  is  turned  by 
the  milled  head  T,  in 
bringing  the  coil  back 
to  zero.  The  scale  is 
marked  in  degrees,  or 
in  400  equal  divisions. 
The  turning  force  of 
the  spring  is  propor- 
tional to  the  angle 
through  which  it  is 
turned.  The  force 
exerted  on  the  coil 
is  proportional  to  the 
product  of  the  cur- 


FIG.  36. 


rents  carried  by  them,  so  that  when  the  coils  are  in  series 
the  square  of  the  current  flowing  is  proportional  to  the 
angle  D  through  which  M  is  turned. 

In  symbols,  7a  =  A'x  D,  that  is,  the  square  of  the  cur- 
rent =  the  number  of  divisions  D  through  which  m  is 


106  TESTING    OF    DYNAMOS    AND    MOTORS. 

turned  x  by  a  constant.  Extracting  the  square  root  of 
both  sides  we  get,  /  =  \/A'  D.  Now  as  A'  is  a  number 
we  can  take  its  square  root,  thereby  placing  it  outside 
the  radical  sign,  where  we  will  call  it  A;  our  formula 
then  reads,  I  —  A  y ' D,  where  A  is  that  current  which 
will  cause  a  deflection  of  one  division. 

The  advantages  of  this  instrument  are  (i)  it  is  a  zero 
instrument,  /'.  e. ,  the  coils  at  the  time  of  taking  a  reading 
always  occupy  the  same  relative  position,  so  that  all 
observations  are  made  under  the  same  conditions;  (2)  it 
is  adapted  to  alternating  current  work  since  the  currents 
in  both  coils  agree  in  phase,  thus  producing  a  resultant 
action  that  is  always  in  the  same  direction;  (3)  no  iron 
being  employed  in  the  construction,  ascending  and 
descending  readings  agree  and  no  averaging  is  necessary. 
The  disadvantages  are:  (i)  the  moving  coil  must  be 
brought  to  zero  before  a  reading  can  be  taken,  so  that 
rapid  variations  of  current  are  not  indicated;  the  current 
must  remain  steady  for  a  time  before  a  reading  can  be 
taken;  (2)  it  is  not  dead-beat,  i.  e.,  the  pointer  does  not 
quickly  come  to  rest,  so  that  readings  cannot  be  taken  rap- 
idly; (3)  its  readings  are  influenced  by  neighboring 
magnetic  fields,'  and  when  the  dynamometer  current  is 
considerable  the  earth's  field  exerts  an  influence,  so  that 
the  instrument  must  be  set  up  with  the  plane  of  the  sus- 
pended coil  at  right  angles  to  the  magnetic  meridian;  (4)  it 
is  not  portable  in  the  ordinary  sense  of  the  word;  (5)  the 
coils  are  exposed  and  liable  to  injury  and  the  swinging 
coil  is  influenced  by  air  currents;  (6)  the  scale  being  grad- 
uated in  degrees  or  arbitrary  divisions  the  instrument  is 
not  direct  reading;  (7)  it  does  not  indicate  the  current's 
direction,  which  is  necessary  in  some  work. 


MEASUREMENT    OF    CURRENT.  107 

In  conclusion  it  may  be  said  that  the  Siemens  dynamom- 
eter is  a  valuable  standard  instrument  when  used  under 
fixed  conditions,  as  in  a  laboratory,  but  there  are  other 
instruments  better  adapted  to  portable  work.  Tests, 
however,  could  be  cited  where  the  dynamometer  has 
given  good  service  under  very  unfavorable  conditions. 
In  a  test  on  a  three-phase  alternator  a  Siemens  dynamom- 
eter was  used  to  check  up  the  readings  of  three  ammeters. 
In  addition  to  the  alternators  ten  direct  current  machines 
were  used  as  auxiliaries,  and  in  the  midst  of  these  was  the 
table  of  instruments.  The  dynamometer  readings  com- 
pared very  favorably  with  those  of  a  newly  calibrated 
ammeter. 

In  reading  small  currents  great  care  must  be  taken 
to  eliminate  sources  of  error  liable  to  modify  results, 
as  the  error  might  in  such  cases  amount  to  a  consider- 
able portion  of  the  whole  current.  With  the  currents 
usually  employed  in  machine  testing  the  results  may 
be  relied  upon  even  under  very  unfavorable  conditions. 
Accompanying  every  instrument  is  a  table  giving  the 
current  value  corresponding  to  each  graduation  of  the 
torsion  head;  this  table  is  determined  by  the  formula 
I  —  A  <J  D,  where  A  is  the  same  for  all  parts  of  the  scale, 
and  can  be  experimentally  found  by  means  of  a  standard 
ammeter  in  series  with  the  dynamometer  to  determine  the 
value  of  I  :  then 


The  table  can  thus  be  checked  up.  It  is  well  to  stan- 
dardize the  dynamometer  from  time  to  time  if  it  is  carried 
around  very  much  or  is  in  constant  use. 

It  may  sometimes  be  desirable  to  connect  the  two  coils 


I08  TESTING    OF    DYNAMOS    AND    MOTORS. 

in  parallel  instead  of  in  series.  In  such  a  case  the  cur- 
rent divides  between  the  two  coils  in  the  inverse  ratio  of 
their  resistances,  and  the  turning  force  exerted  upon  the 
coil  is  proportional  to  the  product  of  the  two  currents. 
The  formula  then  becomes  /  —  A'  ^/ D ',  where  A  has 
a  new  value  different  from  that  with  the  coils  in  series. 
The  deflection  JD'  is  now  due  to  two  factors  which  have 
different  values  also.  By  placing  resistance  in  series 
with  one  of  the  coils  its  current  may  be  varied  and  its 
deflecting  influence  altered.  The  current  in  the  other 
coil  is  also  altered  and  its  deflecting  power  affected. 
When  the  sum  of  the  two  currents  is  constant,  the  total 
deflecting  force  as  represented  by  their  product  will  be  a 
maximum  when  the  currents  are  equal.  Thus  if  the  total 
currents  be  10  amperes  it  may  divide  in  various  ways  as 
given  in  the  table  and  the  maximum  effect  is  seen  to  be 
when  /j  =  1 2  =  5  amperes.  When  this  is  the  case  the 
indication  on  the  dial  will  be  the  same  as  if  the  current 
of  either  coil  were  flowing  through  the  two  in  series;  but 
with  coils  in  series  the  current  in  either  would  be  the 
total  current,  while  when  in  multiple,  under  the  above 
condition,  it  is  but  half;  and  the  indication  on  the  dial 
must  be  multiplied  by  2  to  get  the  true  value  of  the  cur- 
rent flowing. 

7  in  No.  i. 


The  table   shows  that  when  /,  =  /a  the   instrument  is 
most  sensitive,  /'.  e.t  gives  the  largest  deflection  for  a 


7  in  No.  2. 

Tot.  7. 

Deflec  Force. 

2 

10 

8  X  2  =  16 

4 

10 

6  X  4  =  24 

5 

10 

5  X  5  =  25 

6 

10 

4  X  6  =  24 

8 

10 

2  X  8  =  16 

MEASUREMENT    OF    CURRENT.  109 

given  current.  The  advantage  gained  by  the  multiple 
connection  is  to  increase  the  capacity  of  the  dyna- 
mometer without  increasing  the  current  through  its 
coils.  With  an  alternating  current  the  multiple  connec- 
tion is  not  to  be  recommended,  as  the  difference  in  the 
self  induction  and  capacity  of  the  two  coils  may  throw 
the  two  currents  out  of  phase  and  thereby  render  their 
deflecting  effect  less  than  the  product  of  the  two  currents. 

Another  instrument  adapted  equally  well  to  direct  or 
alternating  current  work  is  the  Thomson  or  Kelvin 
balance,  which  has  the  advantage  of  being  more  portable. 
An  objection  to  its  use  save  in  special  work  i*s  its  narrow 
range.  As  it  belongs  rather  to  the  class  of  high  grade 
laboratory  instruments  than  to  commercial  types,  the 
reader  is  referred  to  the  various  laboratory  manuals  for 
its  description  (Carhart  or  Gray). 

We  will  close  our  consideration  of  current  measure- 
ment by  touchingon  the  theory  of  the  electro-dynamom- 
eter used  as  a  measurer  of  work,  /.  e.,  the  wattmeter. 
We  have  as  an  expression  of  the  watts  consumed  in  any 
portion  of  a  circuit,  E  X  /  where  E  is  the  potential 
difference  between  the  ends  of  the  circuit  and  /  the  cur- 
rent flowing.  Now  if  the  heavy  coil  of  an  electro-dyna- 
mometer be  placed  in  the  main  circuit,  the  deflecting 
force  due  to  this  coil  will  be  proportional  to  the  line 
current  /.  If  the  other  coil  be  wound  for  high  resist- 
ance, and  placed  across  the  mains  or  across  that  portion 
of  the  circuit  under  consideration,  its  defecting  force 
will  be  due  to  £,  and  proportional  to  it.  The  resultant 
deflecting  force  must  then  be  proportional  to  E  X  /,  the 
watts. 

For  testing-room  work  the  dynamometer  is  an  invalua- 


110  TESTING    OF    DYNAMOS    AND    MOTORS. 

ble  instrument.  For  commercial  purposes,  however,  a 
wattmeter  is  necessary  that  will  make  a  continuous 
record  of  the  energy  consumed  on  a  circuit,  precisely  as 
a  gasmeter  records  the  consumption  of  gas. 

This  need  is  filled  by  the  various  recording  wattmeters, 
of  which  the  Thomson  recording  meter  will  serve  as  a 
type.  The  instrument  consists  of  a  simple  skeleton 
motor  having  no  iron  in  either  armature  or  field.  The 
fields,  which  consist  of  two  coils  one  on  either  side  of 
the  armature,  are  connected  in  series  with  the  lamps  or 
motor  whose  absorption  of  energy  they  are  to  measure, 
all  the  current  in  use  passing  through  them.  The  arma- 
ture, which  is  of  what  is  familiarly  known  as  the  "Sie- 
mens-drum "  class,  having  a  small  silver  commutator  and 
brushes,  with  silver  contact  pieces,  is  in  shunt  across  the 
line,  like  a  lamp.  It  has  a  considerable  resistance  of 
fine  wire  in  series  with  it,  and  thus  forms  a  "pressure 
coil "  whose  current  varies  with  the  voltage. 

Now  as  the  field  varies  with  the  current,  and  the  arma- 
ture with  the  E.  M.  F.,  the  speed  of  the  motor  will  vary 
directly  with  the  product  of  the  current  and  the  E.  M.  F., 
thus  measuring  the  watts  passing  through  the  circuit. 

In  order  to  secure  accuracy  the  influence  of  friction 
must  be  compensated  for.  This  is  accomplished  by 
taking  off  the  armature  circuit  beyond  the  fields,  thus 
securing  a  slight  constant  field  independent  of  that  caused 
by  the  armature  current  though  the  field  coils.  On  very 
low  loads,  where  friction  is  a  strong  factor,  this  constant 
field  forms  a  considerable  part  of  the  total  field,  while  on 
medium  or  full  load  both  the  friction  and  the  constant 
field  are  but  small  factors,  and  may  be  neglected. 

A  device  is  also  adopted  to  cause  the  armature  to  rotate 


MEASUREMENT    OF    CURRENT.  Ill 

at  a  low  speed,  thus  reducing  the  wear  on  the  instru- 
ment. This  is  very  neatly  accomplished  by  placing  on 
the  armature  shaft  a  thin  disc  of  copper,  which  rotates 
between  the  poles  of  three  permanent  magnets.  The 
fields  of  these  magnets  generate  eddy  currents  in  the 
disc,  and  the  work  thus  done  forms  a  drag  on  the  motor 
proportional  to  the  speed. 

The  indications  of  the  meter  are  shown  in  watt-hours 
on  the  dial,  whose  train  of  wheels  engages  with  a  worm 
on  the  shaft,  thus  connecting  it  with  the  revolutions  of 
the  armature.  The  calibration  of  the  meter  is  accom- 
plished by  moving  the  poles  of  the  permanent  magnets 
toward  or  away  from  the  periphery  or  centre  of  the  disc, 
thus  regulating  its  speed,  until  it  is  synchronous  with  the 
standard  instrument. 


CHAPTER  V. 

MEASUREMENT    OF    ELECTROMOTIVE    FORCE. 

LEAVING  the  subject  of  current  and  its  measurement, 
we  pass  to  the  study  of  that  influence,  electromotive 
force,  which  sets  the  electricity  in  motion  and  produces 
the  current.  An  E.  M.  F.  has  been  defined  as  that  which 
causes  or  tends  to  cause  a  current  flow.  The  word  force 
is  not  here  used  in  its  mechanical  meaning,  for  electricity 
is  not  matter  in  the  usual  sense.  An  E.  M.  F.  is  analo- 
gous to  water  pressure  and  potential  difference  (P.  D.)  is  a 
term  corresponding  to  difference  of  level.  P.  D.  is  inde- 
pendent of  the  current  or  resistance  in  a  circuit  just  as 
the  difference  of  level  between  the  two  ends  of  an  inclined 
pipe  is  independent  of  the  water  flow,  or  of  the  obstruc- 
tion offered.  P.  D.  thus  defined  is  the  total  E.  M.  F. 
of  a  circuit,  and  cannot  be  limited  to  mean  the  fall  of 
potential  between  any  two  points;  as,  for  example,  the 
terminals  of  a  battery.  If  a  battery  develops  a  P.  D.  of  2 
volts,  measured  on  open  circuit  by  a  static  voltmeter, 
there  will  still  be  2  volts  on  closed  circuit,  but  there 
will  now  be  a  loss  of  potential  through  the  battery's 
internal  resistance  and  the  terminal  P.  D.  will  be  less 
than  the  total  E.  M.  F.  The  static  voltmeter,  never  clos- 
ing the  circuit,  actually  measures  the  battery's  total  P.  D. 
An  ordinary  voltmeter  practically  realizes  this  condition 
in  that  its  resistance  is  very  high,  and  the  current  so 
small  that  the  internal  loss  of  potential  can  be  neglected. 


MEASUREMENT    OF    ELECTROMOTIVE    FORCE.  113 

Looked  at  a  little  differently,  the  battery  and  meter  are 
in  series  and  the  total  E.  M.  F.  distributes  itself  accord- 
ing to  the  resistance:  the  meter  resistance  is  so  great 
compared  to  that  of  the  battery  that  the  drop  through 
the  battery  can  be  neglected. 

U'hile  the  total  E.  M.  F.  is  independent  of  resistance  or 
current,  the  ''drop"  between  any  two  points  of  a  closed 
circuit  depends  upon  both.  This  has  its  hydrostatic 
analogy.  If  through  a  pipe  of  uniform  section  a  stream 
of  water  flow,  the  fall  or  decrease  of  pressure  will  be 
uniform,  as  would  be  shown  by  inserting  pressure  gauges 
at  regular  intervals' along  its  length.  If  an  obstruction 
be  placed  in  the  pipe,  as,  for  example,  a  lot  of  sponges, 
the  greatest  fall  of  pressure  takes  place  from  one  side 
to  the  other  of  this  obstruction,  which,  if  so  effectual  as 
to  entirely  stop  the  current  flow,  will  cause  all  gauges  on 
the  pump  side  to  register  full  pressure  and  those  beyond 
the  obstruction  no  pressure  at  all.  Under  a  natural  cur- 
rent flow,  the  gauge  nearest  the  pressure  side  of  the 
pump  registers  highest  and  each  succeeding  gauge  a  less 
amount.  So  in  an  electric  circuit,  if  composed  of  a  uni- 
form homogeneous  conductor,  the  P.  D.  per  unit  of  length 
of  circuit  will  be  everywhere  the  same;  but  if  of  variable 
section  or  composition  the  greatest  drop  will  take  place 
through  the  unit  length  of  highest  resistance.  If  at  any 
point  the  circuit  be  opened  the  resistance  here  becomes  in- 
finite, and  across  it  will  take  place  the  total  P.  D.  of  the 
source.  These  facts  are  expressed  quantitatively  in  Ohm's 
law,  E  —  IR,  where  E  is  the  drop  across  a  portion  of  the 
circuit  of  resistance,  R,  with  a  current,  /,  flowing.  Since 
/is  the  same  for  the  whole  circuit  the  drop,  £,  is  seen  to 
be  everywhere  proportional  to  R.  If  R  =  o,  indicating 


114  TESTING    OF    DYNAMOS    AND    MOTORS. 

the  two  points  to  be  coincident,  E  =  o.  If  7?  increases 
more  and  more  E  does  likewise  and  reaches  a  maximum 
when  R  =  oo,  /.  e.,  when  circuit  is  opened.  In  this 
case,  /  =  o.  When  instead  of  absolute  open  circuit  we 
have  a  voltmeter  across  the  break,  there  flows  a  small 
current.  Upon. this  depends  the  utility  of  voltmeters, 
test-lamps,  magnetos,  etc.,  for  locating  an  open  circuit. 
The  test  consists  in  successively  placing  the  terminals 
across  different  parts  of  the  circuit;  when  the  instru- 
ment includes  the  break,  the  circuit  is  completed  and 
the  lamp,  bell,  or  needle  shows  it.  In  such  testing  it  is 
necessary  to  have  the  resistance  of  the  test  circuit  large 
to  obviate  abnormal  flows  of  current  through  the  devices. 
It  may  seem  unnecessary  to  say  that  a  magneto  must 
not  be  placed  across  a  break  in  a  live  wire,  nor  must  we 
expect  a  voltmeter  or  lamp  circuit  to  give  results  across 
a  break  in  a  dead  one,  unless  the  test  lines  themselves 
include  a  source  of  voltage.  The  voltage  in  use  dictates 
the  meter  or  the  number  of  lamps  that  should  be  in  series. 
With  new  men  there  seems  to  be  a  weakness  due  to 
thoughtlessness,  not  ignorance,  for  placing  a  low  voltage 
meter  across  a  high  voltage  circuit,  or  putting  a  single 
lamp  across  several  hundred  volts,  or  using  a  live  test 
circuit  to  locate  a  fault  in  some  other  live  circuit. 
,  The  commonly  used  sources  of  E.  M.  F.  are  primary 
batteries,  storage  batteries,  and  dynamos.  In  commer- 
cial testing  we  have  most  to  do  with  primary  batteries 
and  dynamos.  Of  batteries,  the  Daniell's  and  Leclanche 
are  most  met  with,  the  former  in  closed,  the  latter  in 
open  circuit  work.  In  closed  circuit  work  the  battery 
carries  a  current  continuously,  but  in  open  circuit  work, 
only  when  the  circuit  is  closed  by  the  button  or  switch. 


MEASUREMENT  OF  ELECTROMOTIVE  FORCE. 


IT5 


—  t— 

—  h 

0 

—  p 

E 

1 

i 

, 

_WOODEH 
COVER 


GLASS  VESSEL 


FOflOUS POTS 


FIG.  37. 


The  Daniell  cell  is  a  double  fluid  one,  having  a  zinc  plate 
clipping  into  zinc  sulphate  and  a  copper  plate  dipping 
into  copper  sulphate  while  a  porous  partition  separates 
the  two  fluids;  more  commonly  each  plate  occupies  a 
porous  cup  containing  the  proper  fluid,  and  both  are 
placed  in  a  vessel  of  zinc  sulphate.  Fig.  37  shows  a 
cell  much  used  as  a 

standard      in     testing  ^  ^b — COV.ECTORS 

rooms.  In  a  glass 
vessel  porous  cups  are 
placed  and  a  wooden 
cover  serves  the  dou- 
ble purpose  of  pro- 
tecting the  solution 
and  holding  the  plates 
in  place.  Three 
points  recommend  the 

cell  as  a  standard  of  E.  M.  F.,  viz.,  (i)  It  has  but 
little  tendency  to  polarize;  (2)  its  voltage  is  nearly 
unity;  (3)  its  E.  M.  F.  is  little  modified  by  temperature 
variations.  To  understand  what  is  meant  by  polariza- 
tion of  a  cell,  we  must  know  that  as  soon  as  the  circuit 
is  closed  and  chemical  action  begins,  hydrogen  gas  is 
set  free  at  the  zinc  plate,  and  if  allowed  to  accumulate 
sets  up  an  E.  M.  F.  opposed  to  that  of  the  cell.  If  the 
current  used  is  small,  the  gas  is  taken  up  chemically  by 
the  sulphate  liquid,  thus  keeping  the  plate  free.  A  well- 
prepared  Daniell  cell  is,  if  carefully  handled,  free  from 
polarization,  and  the  constancy  is  even  improved  by 
allowing  the  cell,  when  not  in  use,  to  send  a  small  cur- 
rent through  a  high  resistance,  thereby  preventing  the 
mixing  of  the  copper  and  zinc  sulphates  by  diffusion. 


Il6  TESTING    OF    DYNAMOS    AND    MOTORS. 

A  single  cell  or  dynamo  or  any  number  of  cells  or 
dynamos  permanently  arranged  may  be  regarded  as  a 
single  cell  or  dynamo  of  increased  voltage  or  current 
capacity,  or  both,  and  will  yield  the  largest  current  on 
short  circuit:  /.  e. ,  any  given  arrangement  of  cells  or 
dynamos,  constituting  a  battery,  will  give  the  most  cur- 
rent when  the  external  or  outside  resistance,  R  —  o,  and 
the  internal  resistance,  r,  alone  limits  the  current  value. 
For 

7=_A_ 


and    if   R    —    o,   corresponding   to   short-circuiting   the 
battery  terminals,  then 


must  have  its  greatest  value  because  r  will  go  into  E 
more  times  than  R  -\-  r  will.  Assuming  no  secondary 
or  armature  reactions  to  exist,  cells  and  dynamos  do  the 
most  work  when  short  circuited,  because  1  is  then  greatest 
and  E  remains  the  same,  and  work  per  second  =  /  E. 
But  none  of  this  work  is  useful;  it  is  wasted  in  internal 
heating  and  is  a  total  loss.  Since  efficiency, 

useful  work 


total  work 

7;  is  here  o  for  there  is  no  useful  work,  though  the  heat- 
ing of  the  cell  shows  that  much  work  is  being  wasted. 
If  from  o,  the  external  resistance  be  gradually  increased, 
the  amount  of  work  done  in  the  external  circuit,  and  with 
it  the  efficiency,  increases.  This  external  or  useful  work 


MEASUREMENT    OF    ELECTROMOTIVE    FORCE.  117 

becomes  a  maximum  when  r  =  R  and  here  ;/  =  50  ^. 
Work  per  second,  as  we  have  seen,  —  EL  If  /  is 
the  same  throughout,  the  work  done  in  different  parts  of 
the  circuit  must  depend  upon  the  distribution  of  E. 
Now  ^distributes  itself  according  to  the  resistance,  and 
since  R  =  ;•  each  must  have  a  drop  equal  to 

E 


and  each  is  the  seat  of  work  per  second  in  amount 
equal  to 

'*?• 

i.  f.,  out  of  a  total  work  per  second  =  /  £.,  one-half  is 
useful  and  //  is,  therefore,  50  %.  Further,  if  by  vary- 
ing R,  the  efficiency  be  made  above  or  below  50  %  the 
useful  work  per  second  will  decrease:  this  is  because 
a  given  change  of  relation  between  r  and  R  has  a 
greater  per  cent,  effect  upon  the  distribution  of  work 
between  them  than  it  has  upon  the  total  work  done. 
Ex. :  Let  E  =  2,  R  =  i,  r  =  i.  Then 

,  2 

/  =  — - —  =  i  ampere. 

The  total  work  done  per  second  is  £  S  =  2X1  =  2 
watts.  The  useful  work  is 

'-  X  I  =  i  X  1  =  1  watt. 
Now  let  R  be  reduced  to  1/2  ohm.     Then, 

r  2  24 

/  =   -      —T-  =  —j-  =  —  ampere, 
i  + 1/2        3/2        3 


Il8  TESTING    OF    DYNAMOS    AND    MOTORS. 

and  the  total  work  per  second  is 

4  8 

-   X  2  =  —  =  22/3   watts, 

<3  o 

and  the  useful  work 

248 
=  -  -   X  —  =  —  watt, 
339 

as  opposed  to  i  watt  before  the  change;  showing  that 
although  the  total  work  has  been  increased  33  %  the 
useful  work  has  been  reduced  n  $,  and  the  total 
effect  has  been  to  not  only  decrease  the  useful  out- 
put per  second  but  to  decrease  rj  as  well.  Let  us  next 
suppose  R  to  be  increased  to  i  1/2  ohm,  everything  else 
being  the  same  as  before. 

2  224 

/  =  -  —7-  =  —. — • —  =  — r   =  -  -  ampere, 

I    +   I    1/2  2/2   +  3/2  5/2  5 

the  current:  4/5  x  2  =  8  '5  =  i  3/5  watt,  the  total  work 
per  second:  3/2  x  4/5  =  12/10  =  6/5  =  drop  across  R9 
and  6/5  x  4/5  =  24/25  watt,  the  useful  work  per 
second.  So  we  see  the  effect  in  toto  has  been  to  decrease 
the  total  work,  increase  the  relative  amount  of  useful 
work,  and  hence  the  efficiency.  That  is  to  say  that 
although  there  is  not  as  much  work  done,  of  that  which 
is  done  a  larger  per  cent,  is  useful. 

We  must  carefully  distinguish  between  the  problem  of 
finding  under  what  condition  a  given  cell  or  battery 
generates  maximum  current,  and  the  problem,  how  must 
a  given  number  of  cells  be  arranged  to  give  maximum 
current  through  a  fixed  external  resistance.  The  answer 
to  the  first  is  R  —  o  to  the  second,  R  =  r.  In  the  first 
case  r  is  fixed,  R  is  not;  in  the  second  case  the  reverse 


MEASUREMENT    OF    ELECTROMOTIVE    FORCE.  119 

is  true.  In  other  words  we  might  take  a  number  of  cells 
and  arrange  them  in  a  certain  way,  and  whatever  way 
that  happened  to  be,  the  battery  would  give  greatest 
current  on  short  circuit;  but  unless  by  chance  we  struck 
the  right  combination,  we  could  rearrange  the  cells  to 
give  a  greater  current  by  fulfilling  the  condition  A'  =  r. 
The  greater  R  compared  to  /-,  the  greater  the  efficiency 
at  which  the  battery  works,  and  when  R  becomes  very 
great  ?;  is  very  nearly  100  £,  but  we  cannot  say  that 
the  efficiency  is  greatest  when  R  —  infinity,  correspond- 
ing to  open  circuit,  for  then  /  is  o,  and  although  the 
battery  is  not  taking  in  or  wasting  any  work,  neither 
is  it  giving  out  any  useful  work,  and  the  expression  for 
efficiency 

useful  work 

»»    — 

total  work 
becomes 


an  indeterminate  quantity. 

A  cell's  internal  resistance  depends  upon  the  density 
of  the  solutions,  the  size  of  the  plates,  their  distance 
apart,  and  the  thickness  and  composition  of  the  porous 
pots.  On  the  other  hand  the  E.  M.  F.  is  independent 
of  dimensions  and  depends  solely  upon  the  nature,  purity, 
and  density  of  the  acids,  and  on  the  nature  and  purity  of 
the  plates.  The  use  of  impure  metals  gives  rise  to  local 
action  detrimental  to  the  E.  M.  F.  and  durability  of  the 
cell.  To  insure  homogeneity  in  the  plates  the  zinc 
should  be  amalgamated,  and  the  copper  electroplated. 
Before  amalgamation  the  zinc  must  be  thoroughly 
cleansed  with  scratch  brush  and  buffer,  then  dipped  for 


120  TESTING    OF    DYNAMOS    AND    MOTORS. 

a  moment  in  sulphuric  acid.  Next  pour  mercury  into  a 
flat  tray  with  dilute  sulphuric  acid,  and  with  a  rag  wash 
acid  and  mercury  together  on  to  the  zinc,  until  its  surface 
is  uniformly  bright.  Where  dirt  spots  exist  the  mercury 
and  zinc  will  not  unite.  After  amalgamation  wash  in 
running  water  to  remove  all  trace  of  acid.  The  copper 
plate  is  prepared  after  the  manner  of  the  voltameter 
plates  already  given.  The  influence  of  solution  density 
and  condition  of  plates  is  as  follows: 

Increase  in  density  of  copper  sulphate  increases 
the  E.  M.  F. 

Increase  in  density  of  zinc  sulphate  decreases  the 
E.  M.  F.* 

Oxidation  in  zinc  surface  decreases  the  E.  M.  F. 

Oxidation  in  copper  surface  increases  the  E.  M.  F. 

The  presence  of  oxide  is  to  be  particularly  guarded 
against  by  electroplating  immediately  before  using  the 
cell,  otherwise  an  error  of  as  much  as  2  %  may  result. 
Fleming  recommends  the  following  as  standard  solutions: 

1.  Copper  sulphate  saturated  at  15°  C.  density  1.2. 

2.  Zinc  sulphate  saturated  at  15°  C.  density  1.2. 
This  combination  will  with  clean  plates  give  an  E.  M.  F. 
of  very  nearly  1.102  volt.     A  second  receipt  is: 

1.  Copper  sulphate  saturated  at  15°  C.  density  i.i. 

2.  Zinc  sulphate  saturated  at  15°  C.  density  1.4. 
Giving  an  E.  M.  F.  of  1.072  volt.     If  the  cell  is  allowed 
to  stand  an  hour  or  so  its  voltage   will   rise  about  .003 
volt,  while  the  presence  of  any  impurity  on  the  plates 
will  lower  it  2  or  3  <fc. 

Says  Professor  Carhart  of  this  cell:  "The  many  pre- 
cautions required  to  insure  a  normal  E.  M.  F.  in  a 

*  Carhart's  Primary  Batteries,  pp.  100,  103. 


MEASUREMENT    OF    ELECTROMOTIVE    FORCE.  121 

standard  Daniell  cell,  on  every  occasion  of  its  use,  are 
more  than  an  offset  to  a  negligible  temperature  coeffi- 
cient in  comparison  with  that  of  a  Clark  cell."  Inas- 
much, however,  as  the  Daniell  cell  can  be  constructed  in 
any  testing  room  while  the  standard  Clark  cell  can  only 
be  procured  by  purchase,  the  former  will  no  doubt  hold 
its  own  as  a  standard  easily  made  and  easily  replaced. 

Where  the  Daniell  is  used  as  a  standard  it  is  well  to 
always  make  up  two  cells,  in  order  that  they  may  be 
checked  up  by  comparison  with  each  other.  If  one  causes 
the  galvanometer  to  show  a  deflection  of  1 10  divisions,  the 
other,  under  similar  conditions,  should  do  the  same;  with 
a  series  connection  and  the  cells  in  opposition  there 
should  be  no  deflection. 

For  open  circuit  work  such  as  bells  and  signals  and  in 
some  zero  methods  of  testing,  the  Leclanche  cell  is  much 
used.  It  is  not  suited  for  general  testing  on  account  of 
its  rapid  polarization  when  giving  a  current.  To  appre- 
ciate this  effect  it  is  only  necessary  to  press  a  bell  button 
for  a  few  moments,  when  the  bell  will  finally  cease  to  ring. 
The  cell  will  however  regain  its  former  K.  M.  F.  if  allowed 
to  stand  for  a  while,  because  the  hydrogen  gas  accumu- 
lated disappears,  and  with  it,  its  back  E.  M.  F.  The 
structure  of  this  cell  is  as  follows:  In  a  glass  jar  contain- 
ing a  solution  of  sal  ammoniac  is  placed  a  zinc  rod  con- 
stituting one  terminal  of  the  cell.  The  other  terminal, 
a  carbon  plate,  is  in  a  porous  cup,  and  is  surrounded  by  a 
mixture  of  powdered  carbon  and  manganese  dioxide;  the 
office  of  the  porous  cup  is  to  hold  the  dioxide  and  carbon 
closely  about  the  carbon  plate.  The  liquid  diffuses 
through  the  cup  and  the  cell  is  ready  for  use.  The  object 
of  the  dioxide  is  to  oxidize  the  hydrogen  as  fast  as  it  is 


122  TESTING    OF    DYNAiMOS    AND    MOTORS. 

liberated  and  thus  to  free  the  cell  from  polarization.  Ex- 
cepting  for  small  currents,  the  action  of  the  manganese 
dioxide  is  not  rapid  enough  to  do  away  with  all  polariza- 
tion, but  a  cell  soon  recovers  itself  when  allowed  to  stand 
idle.  A  milky  color  of  the  solution  indicates  that  the 
salammoniac  needs  renewal,  and  it  is  best  to  entirely 
replace  the  old  solution  with  new.* 

Having  briefly  spoken  of  the  sources  of  E.  M.  F.  we 
may  now  pass  to  its  measurement.  The  galvanometer  is 
the  backbone  of  all  instrumental  work.  In  the  testing 
room  it  is  desirable  to  have  the  instrument  arranged  to 
measure  current,  potential '  difference,  and  resistance. 
To  fulfill  these  varied  requirements,  no  galvanometer  is 
better  adapted  than  a  high  resistance  reflecting  Thomson 
galvanometer  provided  with  suitable  shunts,  directing  mag- 
net and  scale.  Such  an  instrument  is  qualified  to  read  the 
drop  on  field'or  armature,  to  give  insulation  resistance  from 
zero  to  millions  of  ohms,  or  to  indicate  current  values  be- 
tween limitsfixed  onlyby  the  shunt  in  use.  The  requisites 
which  a  good  galvanometer  should  fulfill  are  as  follows: 

(a)  An  astatic  magnetic  system' of  small  weight. 

(b)  A  variable  magnetic  control. 

(c)  Four  coils  of  nearly  equal  resistance  and  effect. 

(d)  High  insulation  and  resistance. 

The  first  is  secured  by  constructing  the  needle  as 
follows  f  :  a  No.  15  aluminum  wire  from  2  1/2"  to  3"  long  is 
flattened  at  both  ends,  and  to  it,  at  a  distance  apart  equal 
to  \he  distance  between  the  centres  of  the  coils,  mica 

*  The  subject  of  Primary  Batteries  is  very  ably  treated  in  a  little  work 
by  Professor  Carhart,  and  the  more  ambitious  reader  is  referred  to  this 
work  for  further  information. 

f  Carhart's  Elect.  Meas.,  p.  145. 


MEASUREMENT    OF    ELECTROMOTIVE    FORCE.  123 

discs  are  attached,  by  means  of  beeswax.  Pieces  of  small 
piano  wire  3/8"  to  1/2"  lung  are  permanently  magnet- 
ized and  are  placed  on  the  back  of  the  discs,  as  shown  in 
Fig.  38,  with  their  poles  opposing  each  other. 
Finally  the  needle  is  suspended  by  a  silk'  fibre, 
and  by  drawing  a  magnet  over  the  system  it  is 
made  perfectly  astatic  (/.  e. ,  so  that  the  yVand  ^V 
poles  exactly  balance).  This  condition  is  indi- 
cated by  the  needle  finally  setting  itself  E  and 
IT  instead  'of  N  and  S.  To  understand  this 
action  we  must  look  more  fully  into  the  mean- 
ing and  advantages  of  an  astatic  combination. 
In  an  astatic  combination  one  needle  has  its 
TV7,  the  other  its  6"  pole  pointing  in  the  same 
direction.  The  result  is  that  any  tendency 
of  one  needle  to  place  itself  in  regard  to  the 
earth's  field  is  opposed  by  the  counter  ten- 
dency of  the  other  needle  to  which  it  is  rigidly 
attached.  The  earth's  field  then  having  an 
equal  but  opposite  influence  on  the  two  needles,  has  no 
influence  on  the  system  as  a  whole,  other  than  to  cause 
them  to  take  up  the -£  and  }V  position;  in  other  words, 
this  peculiar  behavior  is  due  to  the  fact  that  the  needles 
though  of  equal  and  opposite  strength  are  not  perfectly 
aligned,  and  the  resultant  effect  of  the  earth's  field  sets 
them  east  and  west.*  A  perfectly  astatic  balance  is  seldom 
attained  and  is  seldom  needed  in  the  more  practical  lines 
of  work.  A  perfect  balance  deprives  the  needles  of  all 
directing  force  and  makes  the  system  unsteady.  To 
complete  the  suspended  part  a  small  round  mirror  is 
attached  with  wax  to  one  of  the  mica  discs. 

*  See  Ayrton's  Prac.  Elect.,  p.  282,  note. 


1*4  TESTING    OF    DYNAMOS    AND    MOTORS. 

The  directing  magnet,  which  stands  directly  over  the 
coils  and  can  be  either  rotated  or  moved  up  and  down, 
secures  the  (b]  condition  of  variable  magnetic  control. 
The  object  of  this  is  two-fold:  i.  To  render  the  system 
independent  of  the  earth's  field,  or  of  other  disturbing 
fields.  2.  To  change  the  galvanometer's  sensibility,  /.  <?., 
to  alter  the  deflection  which  a  given  current  causes,  and 
thereby  varying  the  galvanometer's  reading  range. 

In  the  testing  rooms  of  large  electrical  works  where 
moving  masses  of  iron  and  charged  fields  of  varying 
intensity  act  as  disturbing  influences  it  is  customary  to 
shield  the  galvanometer  with  several  soft  iron  rings  sur- 
rounding the  coils  in  the  horizontal  plane  of  the  needles. 
Sometimes  the  instrument  is  put  in  an  iron  box  provided 
with  a  hole  through  which  the  mirror  reflects  the  ray. 
The  iron  gathers  to  itself  any  wandering  lines  of  force 
and  frees  the  needle  from  their  influence.  (It  is  for  this 
reason  that  iron  cases  protect  watches  from  magnetism.) 
With  an  unprotected  galvanometer,  care  must  be  taken 
that  masses  of  iron  in  the  immediate  neighborhood  of  the 
galvanometer  are  not  disturbed  during  the  test,  nor  must 
the  directing  magnet  be  touched.  For  if  the  needle  is 
adjusted  to  o  under  influences  which  are  afterward 
changed  the  adjustment  is  destroyed,  and  error  intro- 
duced. Where  the  disturbing  influences  are  very  strong, 
as  in  commercial  testing  rooms,  it  is  found  best  to  have 
the  controlling  magnet  well  down  over  the  needle;  this 
also  lessens  the  error  likely  to  arise  from  want  of  adjust- 
ment in  the  earth's  meridian.  Another  precaution  to  be 
taken  is  to  be  sure  that  the  suspension  fibre  is  not  under 
torsion  when  the  ray  of  light  is  at  o;  on  delicate  instru- 
ments this  torsion  should  be  negligible  since  the  maker 


MEASUREMENT    OF    ELECTROMOTIVE    FORCE.  125 

is  supposed  to  eliminate  it  as  far  as  possible,  by  using 
only  the  finest  of  silk,  or  a  quartz  fibre.  The  presence  of 
error  due  to  torsion  can  only  be  proven  ultimately  by 
making  the  needle  as  sensitive  as  possible,  adjusting<-Aw//y 
in  the  earth's  meridian  and  noting  if  the  deflections  on 
opposite  sides  of  o  are  equal;  provided  other  sources  of 
error  may  be  assumed  to  have  been  eliminated,  the 
inequality  may  be  charged  to  torsion  and,  if  appreciable, 
the  suspension  changed. 

To  facilitate  adjustment  in  the  earth's  field,  the  mag- 
netic meridian  should  be  determined  as  accurately  as 
possible,  and  laid  off  on  the  testing  table;  this  can  be 
done  by  allowing  a  compass  needle  to  come  to  rest,  sight- 
ing along  its  A*  and  6"  poles,  and  projecting  its  direction 
line  on  the  table:  in  doing  this  external  magnetic  influ- 
ences of  local  character  should  be  removed,  and  the 
position  of  all  magnetic  bodies  be  that  in  which  it  is 
intended  the  work  shall  afterward  be  carried  on.  A 
second  -A7' and  S  line  must  now  be  laid  off  at  a  distance 
from  the  first  equal  to  the  intended  distance  between 
needle  and  scale.  These  lines  are  each  laid  off  by  means 
of  the  compass,  and  if  both  observations  have  been  cor- 
rect the  lines  will  be  parallel.  An  /sand  /F  line  must 
now  be  drawn  and  the  galvanometer  needle  placed  over 
one  intersection,  the  zero  of  the  scale  over  the  other. 
In  mapping  out  the  lines  the  tester  must  bear  in  mind 
that  the  aluminum  pointer  on  most  home-made  compasses 
is  at  right  angles  to  the  needle  itself.  It  may  happen 
that  the  mirror  is  not  mounted  exactly  parallel  to  the 
needles  and  the  scale  may  have  to  be  shifted  or  the  lamp 
moved  to  bring  the  ray  to  o.  In  all  galvanometer  work 
vibrations  of  the  needle  due  to  jarring  greatly  hinders 


126  TESTING    OF    DYNAMOS    AND    MOTORS. 

rapid  readings.  The  best  results  are  obtained  by  plac- 
ing the  instrument  on  a  stone  pier  built  up  from  the 
ground,  and  entirely  free  from  the  building.  If  this  is 
impracticable  there  should  be  set  into  the  wall  of  the 
building  a  shelf  long  enough  to  accommodate  the  galvano- 
meter and  scale.  The  wall  chosen  should  have  its  length 
in  the  direction  of  the  meridian,  so  that  the  shelf  may  be 
long  and  narrow.  With  regard  to  the  instrument  itself 
it  is  necessary  that  the  needle  should  be  at  the  centre 
of  the  coil;  if  :t  is  not,  not  only  will  the  coil's  influence 
vary  for  different  positions  of  the  needle,  but  a  more 
evident  annoyance  will  be  a  swinging  motion  of  the  needle 
each  time  the  circuit  is  closed,  due  to  the  needle's  being 
more  powerfully  attracted  to  the  nearer  side  of  the  coil. 
Proper  adjustment  is  secured  by  making  the  fibre  the 
right  length  and  then  accurately  leveling  the  instrument 
by  means  of  screws  provided  for  the  purpose.  On  high 
grade  galvanometers  is  found  a  plumb  line  attachment. 
On  other  instruments  the  eye  alone  is  used  to  centre  the 
needle.  In  regard  to  the  (<r)  and  (d)  requisites  above 
cited:  they  are  in  a  degree  akin  to  each  other.  On  in- 
struments having  symmetrically  disposed  coils,  these 
must  be  electrically  balanced  so  that  their  effects  will  be 
the  same;  and  for  a  given  size  bobbin  and  a  given  size 
wire,  condition  (c)  is  fulfilled  if  the  electrical  resistances 
are  the  same,  for  we  may  then  be  reasonably  sure  that  all 
spools  have  the  same  number  of  turns.  Specification  (d) 
for  high  resistance  is  a  technical  way  of  saying  that 
the  instrument  must  be  wound  with  fine  wire  so  as 
to  get  on  a  great  many  turns,  and  thereby  make  it  very 
sensitive:  for,  to  give  it  a  high  resistance  merely  for  its 
current  limiting  effect,  would  be  much  less  economical 


MEASUREMENT    OF    ELECTROMOTIVE    FORCE.  127 

than  to  introduce  resistance  outside  of  the  coils.  High 
insulation  is  necessary  to  the  successful  operation  of 
any  instrument.  Finally,  there  should  be  adequate 
protection  from  dust  and  moisture.  The  former  is 
secured  by  keeping  the  galvanometer  under  cover;  the 
latter,  by  placing  near  it  a  box  of  chloride  of  lime  or  an 
open  vessel  of  sulphuric  acid,  either  of  which  will  absorb 
moisture.  The  presence  of  dust  not  only  invites  a  short- 
circuit  in  its  more  manifest  forms,  but  by  absorbing 
moisture,  and  affording  a  leakage  path,  affects  the  value 
of  the  shunt  and  thereby  introduces  error. 

Wires  leading  to  and  from  the  galvanometer  should  be 
twisted  together,  so  as  to  neutralize  each  other's  mag- 
netic effect  upon  the  needle.  These  wires  should  also  be 
secured  to  a  stout  insulator  before  passing  to  the  instru- 
ment, thus  securing  it  against  injury  should  the  lines 
receive  an  accidental  jerk.  Convenient  to  the  operator's 
right  hand  should  be  a  reversing  switch,  and  a  key  for 
closing  the  galvanometer  circuit.  It  is  very  convenient 
to  have  the  two  combined  in  one  device. 

The  knack  of  taking  readings  rapidly  on  both  sides  of 
o,  is  acquired  only  by  practice  in  the  use  of  the  key. 
Thus  if  the  ray  be  at  o  and  the  key  be  closed,  the  ray 
swings  immediately  and  the  needle's  inertia  carries  the 
ray  far  beyond  the  true  deflection;  if  the  key  is  kept 
closed  some  time  will  elapse  before  the  ray  comes  to 
rest.  If,  however,  as  soon  as  the  ray  is  well  started  up 
the  scale  the  key  be  opened,  the  needle  stops  almost 
immediately  and  the  ray  begins  to  descend;  at  this 
instant  the  key  may  be  again  closed:  by  repeatedly 
opening  and  closing  the  key  at  the  right  instant  the  ray 
is  soon  brought  to  a  stand.  The  same  tactics  may  be 


128  TESTING    OF    DYNAMOS    AND    MOTORS. 

pursued  in  returning  the  ray  to  o,  and  in  taking  reversed 
readings. 

As  a  precaution  against  danger  in  handling,  as  well  as 
against  error,  it  is  well,  where  high  voltages  are  used,  to 
cover  all  keys  and  switch  handles  with  hard  rubber,  and  to 
mount  the  tester's  stool  on  insulators.  Since  it  is  unsafe 
to  introduce  high  voltage  into  the  galvanometer  circuit,  it 
is  customary  when  measuring  such  voltages  to  use  pro- 
portion lines.  Proportion  lines  are  a  device  by  which 
a  known  fraction  of  the  external  voltage  is  applied  to  the 
galvanometer  circuit.  In  using  this  device  the  operator 
is  still  liable  to  shocks  incidental  to  a  grounded  circuit, 
but  is  not  liable  to  the  full  voltage  by  getting  his  hand 
across  the  key  when  it  is  open.  Again,  by  using  these 
lines  the  higher  voltages  are  read  without  the  necessity 
of  placing  in  the  galvanometer  circuit  very  expensive 
resistance  boxes  to  reduce  the  current  to  a  readable  deflec* 
tion.  It  is  true  that,  outside  high  resistances  must  be 
used,  but  here  it  is  necessary  to  limit  the  current  to  per- 
haps 30  amperes,  while  in  the  galvanometer  circuit  it  must 
not  rise  above  say  30/10,000  of  an  ampere.  The  propor- 
tion lines  consist  of  a  resistance  box  of  such  a  value  and 
capacity  as  to  carry  without  undue  heating  the  current 
sent  through  it  by  the  voltage  to  be  measured.  Across 
a  known  fraction  of  this  resistance  are  connected  lines 
to  the  galvanometer  which  measures  the  P.  D.  included. 
The  total  voltage  is  to  the  P.  D.  measured  as  the  totai 
resistance  is  to  that  included  by  the  lines  or  Vt  :  VQ  \  * 
R  :  r,  whence 


MEASUREMENT    OF    ELECTROMOTIVE    FORCE. 


129 


where  Vg  is  the  P.  D.  measured  by  the  galvanometer.  In 
direct  current  work  where  potential  differences  exceed- 
ing 700  or  800  volts  are  seldom  met  with,  this  method 
can  be  used  throughout  the  whole  range  of  commercial 
work;  higher  voltages,  usually  found  on  arc  machines, 
are  handled  by  increasing  the  resistance,  Rt.  Nor  will 
the  heating  of  the  box,  Rt,  affect  the  result  if  it  is 
made  throughout  of  the  same  material  so  arranged  as 
to  insure  uniform  ventilation.  As  Rt  increases,  due  to 
rise  of  tempera- 
ture, r  does  like- 
wise, and  at  the 
same  rate,  and  re- 
mains the  same 
fraction  of  Rt  as 
when  cold.  If, 
however,  one  part 


FIG.  39. 


be  of  copper  and  the  other  of  German  silver,  or  any  other 
metal  than  copper,  the  device  will  give  correct  results 
only  for  the  temperature  at  which  it  was  set  up,  for  all 
metals  differ  in  the  amount  of  their  resistance  variation 
for  a  given  change  in  temperature. 

It  is  practicable  to  arrange  a  single  proportion  box 
for  reading  over  a  wide  range  of  voltage,  but  it  is  better 
to  have  two  or  more  boxes  standardized  between  their 
limits.  Fig.  39  shows  the  connections  for  a  box 
intended  to  read  any  voltage  up  to  2,000.  For  reading 
up  to  125  switch  A""  is  closed  and  the  other  switches 
remain  open.  For  250,  K"  is  closed,  the  rest  being  open, 
etc.  Since  the  resistance,  off  which  the  galvanometer  G 
takes  the  drop,  remains  constant,  it  becomes  a  smaller 
and  smaller  fraction  of  Rt  each  time  Rt  is  increased,  so 


130  TESTING    OF    DYNAMOS    AND    MOTORS. 

that  a  new  constant  must  be  used  with  each  switch.  Sup- 
pose, for  example,  that  the  resistance  cut  in  by  K'"  is 
1,500  ohms  and  that  the  galvanometer  reads  the  drop  from 
15  ohms.  Suppose  also,  that  by  means  of  the  standard 
cell  the  galvanometer  has  been  set  up  to  read  100  divisions 
for  a  P.  D.  of  i  volt  at  the  terminals  of  its  circuit.  Now 
if  across  1,500  ohms  there  is  a  drop  of  125  volts,  across  15 
ohms,  which  is  i/ioo  of  1,500  ohms,  there  will  be  a  drop 
of  i/ioo  of  125  volts  =  1.25  volt;  further  since  i  volt  will 
deflect  the  ray  100  divisions,  1.25  volt  will  deflect  it 
1.25  x  ioo  divisions  =  125  divisions.  That  is  to  say  the 
scale  is  direct  reading  since  for  every  division  deflection 
there  is  i  volt  applied  to  J?t.  From  proportion,  125  : 
Vt  \  \  1,500  :  15;  or  Vg  —  1.25  volt  as  before.  Also  i 
volt  :  1.25  volt  \\  ioo  divisions  :  x.  Whence  x  = 
125  divisions  as  explained  above.  Next  suppose  the 
voltage  under  measurement  to  be  250  and  suppose  that 
K'1  cuts  in  an  additional  1,500  ohms.  The  galvanom- 
eter circuit  has  not  been  disturbed  and  we  now  have, 
250  :  Vg\\  3,000  :  15;  or  Vg  —  1.25  the  same  as 
before,  because  although  we  have  doubled  Vt  we  have 
also  doubled  Rt,  and  since  the  resulting  current  is  the 
same  so  also  must  the  drop  across  the  given  resistance,  r, 
be,  and  hence  the  deflection  125.  But  we  know  (by 
mean  of  a  voltmeter,  if  necessary)  that  Vt  =  250;  i 
division  therefore  means  2  volts  on  the  line  and  the  scale 
is  no  longer  direct  reading,  but  its  indication  must  be 
multiplied  by  2;  2  is  the  constant  for  the  250  volt 
lines.  In  delicate  measurements  on  such  a  box  care  must 
be  taken  not  to  introduce  error  by  using  the  125  volt 
lines  a  long  while  and  suddenly  switching  in  the  250  volt 
lines;  for  the  galvanometer  then  reads  the  drop  from 


MEASUREMENT    OF    ELECTROMOTIVE    FORCE. 


a  combined  cold  and  warm  total  resistance,  which  is  not 
what  it  may  ordinarily  be  supposed  to  be.  It  is  desir- 
ble  to  have  r  (15  ohms  in  the  above  case),  a  sub-multiple 
by  100,  200,  300,  etc.,  of  the  successive  values  of  R. 
Thus  if  r  —  15,  Rx  ==  1,500;  R^  =  3,000;  R^  --  4,5°°, 
etc.  Then  if  the  galvanometer  is  set  up  to  read  100 
divisions  per  volt  the  scale  reading  need  only  be  multi- 
plied by  i,  2,  3,  etc.,  to  give  the  line  voltage. 

A  neat  way  to  secure  a  direct  reading  scale  is  to  have 
several  scales  ruled  one  above  the  other  on  the  same 

card,  so  mounted  as  to  slide       

up  and  down  so  as  to  bring 
the  ray  on  a  level  with  the 
scale  desired  to  be  used. 
Each  scale  should  be  num- 
bered to  read  volts  direct, 
and,  to  avoid  confusion  in 
reading,  colored  inks  can  be 
used  in  graduating  the  several  scales.  This  is  illustrated 
in  Fig.  40,  whose  scale  was  prepared  for  the  proportion 
box  described  above  and  in  which  the  ratio  of  r  to  R 
was  successively  i/ioo,  1/200,  and  1/300. 

One  proportion  box  can  be  easily  calibrated  by  means 
of  another;  thus  suppose  it  is  desired  to  calibrate  a  1,000 
volt  box  by  means  of  a  125  volt  box.  First,  by  means 
of  the  125  volt  box,  adjust  the  line  voltage  to  100  volts; 
then  apply  this  known  voltage  of  100  to  the  terminals  of 
the  1,000  volt  box,  and  observe  the  deflection;  we  can 
now  do  one  of  two  things:  either  rroject  the  100  volt 
scale  graduation  of  the  lower  scale  onto  the  high  volt 
scale  and  there  mark  it  100,  or,  when  practicable,  we  can, 
by  shifting  the  connections  Df  the  galvanometer  leads, 


FIG.  40. 


132  TESTING    OF    DYNAMOS    AND    MOTORS. 

readjust  the  resistance  r  included  by  the  galvanometer, 
till  a  deflection  of  10  is  obtained  on  the  125  volt  scale. 
This  is  then  projected  to  the  upper  scale  and  marked  100. 
The  latter  method  is  preferable  as  the  readings  in  the  last 
case  have  simple  ratios  to  each  other.  Next  secure  a 
reading  of  200  on  the  125  volt  box,  and  apply  the  200 
volts  as  before  to  the  1,000  volt  box.  The  higher  scale 
reading  should  now  be  twice  its  original  value,  and  any 
departure  indicates  an  error,  most  probably  in  observa- 
tion. Having  secured  two  points,  the  rest  of  the  scale 
can  be  divided  into  equidistant  graduations.  The  value 
of  r,  once  determined,  should  not  be  changed  even 
though  R  is  varied.  It  is  well  to  have  r  composed  of 
double  rather  than  single  wire,  for  should  a  single  wire 
burn  off  or  break,  the  galvanometer  would  be  thrown 
into  the  main  circuit,  instead  of  being  a  shunt  to  it,  there- 
by inviting  injury  to  the  instrument. 

In  using  high  resistance  boxes  care  must  be  taken  that 
they  are  not  exposed  to  moisture,  for  the  adjustment  of 
a  dry  day  may  prove  to  be  useless  on  a  wet  one,  as  the 
resistance  becomes  practically  short-circuited  through 
leakage  paths,  and  it  may  be  impossible  to  reduce  the 
deflection  to  a  readable  amount.  If  comparison  with  the 
standard  cell  shows  such  a  condition  to  exist,  the  boxes 
may  be  subjected  to  the  mild  heat  of  a  couple  of  incan- 
descent lamps  until  under  normal  conditions  the  ray  shows 
the  proper  deflection  for  a  standard  voltage. 

When  using  a  galvanometer  the  tester  must  remove  from 
his  person  all  magnetic  articles  such  as  knives,  watches, 
keys,  etc.  A  not  infrequent  source  of  annoyance  often 
overlooked  is  the  shifting  of  iron  window  weights,  and 
with  high  grade  laboratory  instruments  the  magnetic 


MEASUREMENT    OF    ELECTROMOTIVE    FORCE.  133 

disturbances  coincident  with  auroral  displays  render 
observations  impossible.  These  precautions  should  be 
particularly  observed  when  the  instrument  is  adjusted  to 
its  maximum  sensitiveness,  as  it  is  for  delicate  work. 
Ordinarily  the  directing  force  of  the  earth's  field  is  weak 
enough  to  allow  sufficiently  large  deflections,  but  if  neces- 
sary it  can  be  weakened  to  any  degree  by  placing  the 
directing  magnet  with  its  N  and  S  poles  opposing  the 
earth's  field,  and  at  the  proper  distance,  determined  by 
trial,  above  the  instrument.  If  the  magnet  on  reversal 
be  placed  too  near  the  needle,  it  will  not  only  neutralize 
the  earth's  field  but  establish  one  of  reversed  polarity,  a 
condition  indicated  by  the  needles  turning  halfway  round 
in  obedience  to  the  magnet's  attraction.  It  may  happen 
that  the  magnet  is  too  strong  even  when  at  its  highest 
point  on  the  supporting  rod;  in  such  a  case,  either  the  rod 
must  be  lengthened,  a  weaker  magnet  used,  or  a  second 
magnet  so  disposed  as  to  help  the  earth's  field. 

The  wiring  of  a  galvanometer  service  is  a  field  for  judg- 
ment and  care;  on  the  switchboard  are  brought  out  the 
terminals  of  all  circuits  used  in  measuring,  and  in  a  cen- 
tral position  should  be  found  the  galvanometer  terminals. 
All  live  wires  should  be  placed  on  one  side  of  the  gal- 
vanometer terminals,  all  dead  wires  on  the  other;  the 
galvanometer  terminals  being  understood  to  include  the 
galvanometer  circuit  complete  resistances,  switches,  etc. 
Potential  lines  should  never  be  used  for  measuring 
resistance  nor  resistance  lines  as  potential  lines.  Failure 
to  observe  this  rule  will  result,  sooner  or  later,  in  injury 
to  the  galvanometer  or  tester.  In  plugging  in  proportion 
lines  be  careful  that  the  lines  including  r,  and  not  R,  go  to 
the  galvanometer;  confusion  will  result  not  only  in  pos- 


134  TESTING    OF    DYNAMOS    AND    MOTORS. 

sibly  burning  out  r,  but  in  subjecting  the  galvanometer  to 
the  total  line  voltage  with  probably  fatal  results.  As  a 
certain  preventive  of  confusion  the  plugs  from  the  R 
terminals  can  be  made  so  that  they  cannot  go  into  the  gal- 
vanometer connection  plate.  Galvanometer  and  switch- 
board lines  should  be  placed  on  porcelain  insulators  to 
avoid  leakage,  which  will  otherwise  seriously  modify  re- 
sults when  measuring  insulation.  Rubber-handled  flexible 
cables  should  be  used  in  connecting  the  galvanometer  to 
the  various  shop  lines.  In  making  switchboard  connec- 
M  N  tions,  plug  in  the  live 

r~~^~]-*-/^y—r~~{        r~ n-^/^Y— r~~l     blocks  last;  in  breaking 
\^  i#'\  y(r/  */   E      connections,      unplug 

^"\       .^^^        .^  the    live    blocks  first; 

^    '    JUUUUL,   ^*  tnen    should    a    cable 

drop    from    the   hand 

FlG-  4I-  there  is  little   danger 

of  shock  or  short  circuit.  For  similar  reasons,  it  is  well, 
in  making  new  connections,  to  clear  the  switchboard  of 
those  used  in  a  previous  test.  And  in  changing  connec- 
tions never  plug  a  live  wire  or  a  grounded  wire  into  one  side 
of  a  circuit  unless  its  other  side  is  free.  Thus  suppose 
(Fig.  41)  dynamo  N  to  be  sending  current  through  resist- 
ance R;  suppose  further  that  in  order  to  accommodate  a 
fellow  tester  who  is  in  a  hurry,  we  are  to  transfer  R  to 
dynamo  M.  Suppose,  as  is  too  frequently  really  the  case, 
N.  to  be  accidentally  grounded  at  g  and  M  at  g';  the 
result  of  connecting  B  and  C  or  A  and  D  would  be  to 
ground  both  sides  of  N  in  the  first  case  and  M  in  the 
second,  causing  a  dead  short  circuit.  The  safest  way  to 
avoid  trouble  is  to  disconnect  N  entirely  before  connect- 
ing M :  however,  if  it  so  happens  that  the  tester  gets  the 


MEASUREMENT    OF    ELECTROMOTIVE    FORCE.  135 

grounded  sides  together  there  will  be  no  demonstration. 
The  above  frequently  arises  in  the  hasty  and  apparently 
complicated  wiring  of  large  testing  rooms. 

We  are  now  ready  to  set  up  the  galvanometer  to  read 
voltage.  All  shunts  are  removed  and  the  magnet 
secured  midway  on  its  post.  The  galvanometer  is  then 
placed  in  position  at  the  intersection  of  one  of  the  A"  S 
and  the  E  IV  lines  already  laid  out.  It  is  also  leveled, 
and  with  the  eye  aligned  approximately  in  the  meridian. 
The  scale  and  lamp  are  set  in  place  and  a  clear  ray 
secured.  We  next  adjust  the  coils  to  the  meridian;  this 
is  done  by  removing  the  magnet  and  allowing  the  ray  to 
come  to  rest  under  the  earth's  influence  alone.  The  scale 
is  then  moved  till  the  ray  rests  on  o.  The  galvanom- 
eter is  now  slightly  rotated  on  a  vertical  axis  until  the 
deflection,  due  to  a  small  fraction  of  a  cell's  voltage,  is 
the  same  on  both  sides  of  o.  This  low  voltage  can  be 
obtained  by  placing  a  piece  of  wire  across  the  cell's  termi- 
nals and  taking  the  drop  from  an  inch  or  more  of  it. 
The  magnet  is  now  replaced  and  the  ray  brought  to  zero 
under  its  influence,  by  means  of  the  worm  and  screw 
generally  supplied  for  that  purpose. 

The  next  step  is  to  determine  what  deflection  a  known 
voltage  at  the  galvanometer  circuit  terminals  will  pro- 
duce. It  is  always  best  to  make  the  deflection  100 
divisions  per  i  volt  or  .1  volt  or  .01  volt,  etc.,  accord- 
ing to  the  sensitiveness  of  the  galvanometer;  the  read- 
ings are  thus  much  simplified.  For  ordinary  commercial 
testing  the  standard  Daniell  cell  is  used,  but  for  more 
responsible  work  it  is  customary  to  at  least  compare  the 
home-made  standard  with  some  guaranteed  cell  such  as 
the  Clark  or  Carhart-Clark  standard  cell.  These  cells 


136  TESTING    OF    DYNAMOS    AND    MOTORS. 

are  made  up  in  lacquered  brass  and  ebony  finish,  and 
are  provided  with  a  thermometer  to  facilitate  accurate 
temperature  corrections.  Their  temperature  correction 
is  negative;  /.  £.,  the  E.  M.  F.  falls  as  the  temperature 
rises.  Another  standard  cell  is  the  Calomel,  invented 
by  Von  Helmholtz.  The  E.  M.  F.  of  this  cell  increases 
as  the  temperature  rises,  making  its  temperature  cor- 
rection positive.  This  makes  possible  a  combination  of 
Clark  and  Calomel  cells  free  from  temperature  correction. 
Such  sets  of  cells  self-correcting  between  certain  tem- 
peratures can  be  secured  from  the  manufacturers.  It  is 
fatal  for  such  cells  to  be  short-circuited,  because  the 
current  decomposes  the  paste  of  which  the  cell  is  made. 
To  preclude  such  a  mishap  there  is  placed  inside,  and 
in  permanent  connection  with  the  cell,  a  high  resistance 
coil.  This  coil  does  not  appreciably  affect  the  cell's 
terminal  voltage,  because  as  a  rule  the  cell  is  used  only 
in  zero  methods,  and  when  it  does  send  a  current  it  is 
through  such  very  high  resistance  that  that  within  the 
cell  can  be  neglected. 

The  Clark  cell  has  an  E.  M.  F.  of  1.434  volt  at  15°  C, 
and  this  E.  M.  F.  decreases  .00115  volt  per  degree  C. 
rise  of  temperature  up  to  30°  C.  and  increases  at  the 
same  rate  below  15°  C.  The  cell  should  never  be  used 
at  temperatures  below  10°  C.  or  above  30°  C.  The 
temperature  coefftciency  of  the  Carhart-Clark  cell  is 
about  y2  that  of  the  Clark,  while  that  of  the  Calomel 
is  -j-  .000094  volt  per  degree  C.  above  15°  C.  up  to 
30°  C. :  it  is  about  -fa  of  that  of  the  Clark  cell;  t.  e.,  the 
Clark  cell's  E.  M.  F.  decreases  .00115  vo^  Per  degree 
rise  while  the  Calomel  cell  increases  .00115  volt  -=-  n  = 
.000104  Per  v°lt  degree  under  similar  conditions.  From 


MEASUREMENT    OF    ELECTROMOTIVE    FORCE.  137 

this   it    follows   that    n  Calomel   cells   in   series  with  i 
Clark  will  give  a  self-correcting  battery. 

For  most  forms  of  industrial  testing  a  Daniell  cell 
gives  satisfaction.  Where  a  finer  standard  is  available 
the  E.  M.  F.  of  the  Daniell  should  be  compared  with  it, 
and  then  used  as  a  secondary  standard.  Assuming  this 
to  be  the  case,  and  the  Daniell's  E.  M.  F.  to  be  1.07 
volt,  we  now  place  the  galvanometer  with  its  resistance 
boxes,  containing  100,000  ohms,  constant,  and  10,000 
ohms,  variable,  in  series,  and  no  shunt;  the  directing  mag- 


s 

FIG.  42. 

net  and  resistances  are  adjusted  until  a  deflection  of  107 
is  obtained.  This  will  mean  i  division  per  .01  volt,  and 
i  volt  per  100  divisions.  With  a  scale  of  250  divisions  on 
either  side  of  o,  anything  over  2.5  volts  will  throw  the 
ray  off  the  scale.  For  voltages  higher  than  this  the 
galvanometer  must  be  shunted  or  the  proportion  lines 
used.  On  high  voltages  the  shunt  itself  is  liable  to 
injury  unless  the  extra  resistance  in  circuit  with  the 
galvanometer  at  least  equals  that  given  above.  At  all 
events  be  certain  that  the  shunt  shunts  only  the  gal- 
vanometer and  not  the  boxes  also,  for  this  would  place 
the  shunt  across  the  line  and  a  burn-out  would  follow. 
Provided  galvanometer  and  shunt  are  both  in  series  with 
a  high  resistance  as  in  Fig.  42,  the  danger  of  short  cir- 
cuit is  removed  and  the  shunt  may  be  safely  used.  With 
the  1/99  shunt  a  voltage  of  100  would  give  a  deflection  of 


138  TESTING    OF    DYNAMOS    AND    MOTORS. 

100,  because  with  no  shunt  i  volt  gives  100  divisions;  with 
the  1/99  shunt  only  i/ioo  of  the  current  goes  through 
the  galvanometer  and  i  volt  will  give  but  i/ioo  of  the 
deflection  that  it  gives  without  the  shunt,  or  i/ioo  of  100 
divisions  =  i  division.  Therefore  100  volts  would  give 
a  deflection  100  times  that  due  to  i  volt  or  100  divisions; 
or  the  galvanometer  is  direct  reading  and  using  the  1/99 
shunt  is  equivalent  to  using  the  125  volt  proportion  lines. 
With  the  shunts  any  voltage  can  be  read  up  to  2,500, 
using  the  1/999  Shunt.  By  using  more  than  one  shunt 
at  a  time,  not  a  general  practice,  this  range  is  somewhat 
increased.  With  a  combination  of  shunts  and  propor- 
tion boxes  the  carrying  capacity  of  R  would  alone 
limit  the  range  of  the  galvanometer. 

One  of  the  most  useful  applications  of  the  above  "  set- 
ting up  "  with  a  Daniell  cell  is  in  calibrating  commercial 
voltmeters;  the  voltmeters  are  arranged  on  a  table  at 
a  safe  distance  from  each  other,  and  by  means  of  their 
push  buttons  successively  connected  in  multiple  with  the 
galvanometer  circuit.  Note  that  we  say  galvanometer  cir- 
cuit, because,  were  we  using  proportion  boxes,  the  meters 
would  be  placed  in  multiple  not  with  G  but  with  R,  the 
resistance  to  which  the  line  voltage  is  applied.  A  read- 
ing is  adjusted  on  the  galvanometer,  and  its  value  marked 
on  the  scales  of  the  meters.  All  the  meters  are  success- 
ively graduated  for  each  galvanometer  reading  until 
their  entire  scale  is  calibrated.  As  fast  as  the  ranges  of 
the  lower  reading  meters. are  attained  the  instruments 
are  cut  out  of  circuit.  Instruments  under  calibration 
should  be  slightly  tapped  with  the  finger  to  release  the 
needle  should  it  be  inclined  to  stick.  In  some  types  of 
voltmeter  it  is  unnecessary  to  use  the  full  voltage  to 


MEASUREMENT    OF    ELECTROMOTIVE    FORCE.  139 

calibrate  the  scale,  there  being  provided  what  is  called 

a   calibration   coil.     Its    principle    is    seen    in    Fig    43. 

A  B  C 'is   the    movable    coil   carrying   the  needle.     In 

series   with    this   coil   is  the 

usual     high     resistance     R; 

coming     clown      to     binding 

post  No.   2    is  a   lead   which 

includes    but   a    fraction,    ;-, 

of   the    total    resistance    R. 

The   calibrating  coil  is    that 

part  of  R  included  between  FlG-  43> 

posts  No.  2  and  No.  3.     From  No.  i  to  No.  3  is  the  work- 

ing  coil.     The  principle  is  that  of  the  proportion  box. 

r  is  so  related  to   R,  that  when  a  specified  fraction  of 

the  voltage   which    applied   to  i  and  3  gives  a   certain 

deflection,   is  applied   to    2  and   3,  the   same  deflection 

obtains.     This    means   that  in   either   case   the    current 

through  A  B  C  is  the  same  and  hence  the  deflection  is 

the  same.     Let  i  be  this  current  and  let  E  and  e  be  the 

voltages  respectively  necessary  to  apply  to  R  and  r  to 

produce  /;  then 

i  =  -f-  =  -  °r  •£  :   R\\c  :   r 
K         r 

whence  E  \  e  \\  R  :  r.  Hence  if  r  =  R  -r-  10,  then 
must  e  —  E  -f-  10,  and  by  applying  this  lower  voltage  to 
the  calibrating  coil,  r,  the  whole  scale  can  be  marked  off. 
Since  all  measurements  of  E.  M.  F.  with  the  galvan- 
ometer depend  upon  the  accuracy  of  the  standard  cell, 
great  care  should  be  used  to  preserve  this  accuracy. 
The  best  methods  are  on  this  account  zero  methods  which 


140  TESTING    OF    DYNAMOS    AND    MOTORS. 

involve  no  current  flow,  and  which  are  free  from  tor- 
sional  and  other  disturbances  incidental  to  deflectioa 
methods.  Fig.  44  illustrates  a  zero  method  due  to  Pog- 
gendorf.  B  generates  the  E,  M.  F.  to  be  measured 

and  it  sends  a  small  current 
through  RR.  Fis  a  stand- 
ard cell  in  series  with  which 
is  a  low  resistance  galvanom- 
eter G,  a  variable  high  resist- 
FlG-  44'  ance  r,  and  switch,  K.  One 

terminal  of  the  galvanometer  circuit  is  fixed  at  R,  and 
the  other  is  free  to  slide  along  R  R .  Fis  opposed  ia 
polarity  to  B.  Resistance  R  R  is  an  exposed  German- 
silver  wire  of  known  length  and  uniform  cross-section, 
mounted  on  a  board  graduated  in  inches  or  cms. 
The  cross-section  is  uniform  if  the  P.  D.  per  unit  of 
length  is  everywhere  the  same.  Should  it  be  not  the 
same,  the  resistance  of  each  unit  length  must  be  found 
and  marked  on  the  underlying  scale.  With  r  in  circuit 
P  is  moved  along  R  R  till  ^Tcan  be  closed  without  affect- 
ing G.  r  can  now  be  gradually  cut  out  and  a  finer  ad- 
justment secured.  The  condition  then  existing  is  this: 
From  R  to  P  is  a  certain  P.  D.  which  by  trial  is 
made  equal  to  that  of  V.  Since  the  drop  of  potential  is 
proportional  to  the  resistance,  the  drop  from  RtoP  bears, 
the  same  relation  to  the  total  drop,  as  resistance  R  P 
does  to  the  total  resistance.  But  the  total  drop  is  the 
voltage  of  B,  and  the  total  resistance,  neglecting  that 
of  B,  is  R  R'.  Therefore 

Voltage  R  to  P  :  Voltage  of  B  \  \  Resistance  RP  : 
Resistance  R  R'  or  Ey  :  E*\\  Res.  R  P  :  Res.  R  R'\ 
whence 


MEASUREMENT    OF    ELECTROMOTIVE    FORCE.  14! 

Res.  R  R' 
*  ~  Res.  R  P 

If  the  cross-section  of  R  R  is  uniform,  the  resistance  of 
each  of  the  parts  into  which  R  R  may  be  divided  will  be  pro- 
portional to  its  length;  therefore  ZiV  '.  E^\\  R  P"  \  R  Rn 
and 

£B  =  £v  x 


This  zero  method  of  comparison  is  used  to  calibrate 
high  grade  voltmeters.  Any  number  of  meters  can  be 
put  in  multiple  with  R  R.  j9's  voltage  is  gradually 
raised,  measured  at  each  step,  and  marked  on  the  dials 
of  the  meters  under  test.  This  device  has  another  very 
useful  testing-room  application,  nor  should  it  be  confined 
to  testing-room  practice.  If  we  know  F's  voltage  and 
R  ^"s  resistance,  or  if  of  uniform  section,  R  J?"s  length, 
/>'s  position  for  any  value  of  B  can  be  calculated  from 
the  formula, 


s  E.  M.  F.  =  x  F's  voltage; 

or 


If  £v  =  i  and  R  R'  =  100  where  must  P  be  for  E^  =  50 
volts  ? 

RP  =  ~XR£'  =  —  X  ioo  =  2  inches 
E*  5° 

or  cms.,  as   the  case  may  be.     In    this   way   the   whole 
distance   can   be    laid    off,    and   any   desired    impressed 


142  TESTING    OF    DYNAMOS    AND    MOTORS. 

E.  M.  F.  can  be  secured  by  placing  P  on  the  proper 
point  and  varying  £B  till  balance  obtains.  The  chief 
liability  to  error  lays  in  the  cell's  temperature  variation. 
In  working  with  any  slide  wire  device  care  must  be 
taken  that  the  slide  P  does  not  cut  or  scrape  the  wire 
and  thereby  introduce  error  by  diminishing  R  fi"s  cross- 
section.  To  avoid  having  an  inconvenient  length  of 
wire  in  R  R'  a  resistance  box  is  often  used  in  connection 
with  it  and  sometimes  entirely  replaces  it.  The  objec- 
tion to  a  box  is  that  R  R' 


— H — ©-^K 


and  R  P  must  be  ex- 
pressed in  ohms  and  not 
in  lengths,  whereas  the 
delicacy  of  the  method 
FIG.  45.  rests  largely  on  the 

fact   that   a  -length    can 

be  more  easily  and  accurately  measured  than  a  resist- 
ance: furthermore  the  temperature  of  the  exposed 
wire  will  probably  be  lower  than  that  of  the  box; 
this,  however,  is  of  minor  importance  as  the  current 
should  not  be  large  enough  to  raise  the  temperature  of 
any  part  of  the  circuit.  When  boxes  alone  are  used, 
as  delicate  a  balance  cannot  be  secured  since  points 
between  plugs  are  not  considered.  Fig.  45  gives  con- 
nections for  the  box  method.  Here,  instead  of  the  slide 
wire,  two  boxes  are  used,  and  balance  effected  by  vary- 
ing the  resistance  in  each.  The  total  resistance  of  both 
boxes  becomes  R  R'  and  that  of  r,  R  P.  The  same 
equations  hold.  To  facilitate  calculation  it  is  con- 
venient to  keep  rl  -f  ra  constant;  this  is  done  by  plug- 
ging as  much  resistance  into  one  box  as  is  taken  out  of 
the  other.  There  is  no  limit  to  the  voltage  readable 


MEASUREMENT    OF    ELECTROMOTIVE    FORCE.  143 

by  this  method,  the  single  restriction  being  that  A'  £' 
be  always  sufficient  to  prevent  E^  from  generating  too 
large  a  current.  In  dealing  with  very  high  voltages 
"multipliers"  are  used.  The  same  idea  is  utilized  in 
the  so-called  "multipliers"  supplied  (by  request)  with 
Weston  voltmeters,  to  increase  their  reading  range. 
These  multipliers  consist  of  a  resistance  box  with  a 
known  simple  ratio  to  the  meter's  resistance  and  is 
placed  in  series  with  it.  To  get  the  impressed  voltage 
the  deflection  must  be  multiplied  by  a  number,  known  as 
the  "constant,"  stamped  in  a  conspicuous  place  on  the 
case.  Since  it  is  desirable  that  the  constant  be  a  simple 
number  the  ratio  of  resistances  must  be  a  simple  one; 
a  box  suitable  as  a  multiplier  for  one  meter  will  not  be 
conveniently  so  for  most  others,  the  constant  only 
holding  good  for  meters  of  the  same  resistance.  The 
same  number  is  therefore  stamped  on  meter  and  multi- 
plier. Let  Rv  be  the  voltmeter's  resistance  and  Rm 
that  of  the  multiplier.  If  ^v  =  Rm  the  same  drop  will 
take  place  across  each,  and  we  know  the  meter  indi- 
cates 1/2  the  impressed  voltage,  so  that  the  constant  is  2. 
If  Rm  =  2  Rv,  or 


the  drop  across  the  meter  is  1/2  that  across  the  multi- 
plier and  1/3  the  total  drop  and  the  constant  is  3.  In 
this  case, 

2_RL±_XI\ 

Jtv)- 

and  this  is  the  formula  used  in  determining  the  constant 


144  TESTING    OF    DYNAMOS    AND    MOTORS. 

when  the  multiplier  is  made.     The  general  expression  is 


=  n 


where  n  is  the  constant,  and  the  impressed  E.  M.  F. 
equals  n  times  the  indicated  voltage. 

If  there  is  no  standard  multiplier  convenient  any  box 
of  known  resistance  can  be  used  if  the  meter  resistance 
is  known.  The  constant  n  will  not  in  this  case  be  a 
simple  number,  unless  it  so  happens. 

The  above  principles  have  been  applied  in  producing 
an  instrument  which  can  be  used  both  as  an  ammeter 
and  a  voltmeter.  A  paper  read  before  the  American 
Institute  of  Electrical  Engineers,  March  17,  1894,  by 
E.  G.  Willyoung,  describes  such  an  instrument. 


CHAPTER  VI. 

MEASUREMENT    OF    RESISTANCE. 

HAVING  outlined  the  methods  of  current  and  E.  M.  F. 
measurement,  we  pass  to  that  of  resistance,  the  Last  of 
the  three  factors  found  in  Ohm's  law.  Resistance  is  that 
which  checks  back,  or  hinders  the  flow  of  current  through 
.a  conductor.  Its  analogue  in  mechanics,  or  rather 
hydraulics,  is  friction.  The  presence  of  resistance  is 
looked  upon  as  an  unavoidable  evil,  and  generally  such  is 
the  case;  its  absence,  however,  would  be  sometimes  incon- 
venient, for  resistance  alone  limits  the  current  which  an 
E.  M.  F.  shall  send,  and  should  a  circuit's  resistance  ever 
become  zero,  the  smallest  E.  M.  F.  conceivable  could 
•send  through  it  a  current  of  infinite  value.  Take  a  given 
circuit  and  suppose  its  resistance  to  be  gradually 
decreased.  The  current  produced  by  a  given  E.  M.  F. 
is 


and  as  R  grows  smaller  and  smaller  the  fraction 

R 

grows  larger  and  larger  until,  when  R  becomes  zero, 

7-  — 
o 

145 

• 


146  TESTING    OF    DYNAMOS    AND    MOTORS. 

or  infinity,  written,  oo  .  As  soon  as  a  current  flows  in  a 
wire  lines  of  force  spread  out  from  the  wire,  and  in  doing 
so  cut  the  circuit  at  different  points,  generating  therein 
an  E.  M.  F.  opposed  to  that  which  urges  the  current. 
The  impressed  E.  M.  F.  has  then  to  oppose,  and  do  work 
against  this  opposing  tendency,  until  when  the  current 
reaches  a  steady  value,  the  lines  of  force  cease  to  move, 
there  is  no  more  self-induction,  and  the  field  is  steady. 

We  see  then  that  even  had  wires  no  ohmic  resistance  it 
would  require  work  to  bring  a  current  up  from  zero  to 
any  given  value.  Were  all  conductors  without  resistance 
a  current  once  set  up  would  continue  forever:  there 
being  nothing  to  stop  it. 

There  is  an  interesting  application  of  this  idea  in  the 
theory  of  magnetism.  If  we  assume  a  molecule  of  mag- 
netic substance  to  be  of  zero  resistance,  and  further 
assume  that  by  some  means  a  current  has  been  set  up 
around  the  molecule,  this  current  would  continue  for- 
ever. The  molecule  would  then  be  a  permanent  electro- 
magnet. This  is  one  of  the  theories  advanced  to  explain 
magnetism. 

The  study  of  resistances  falls  under  two  heads :  i.  The 
resistance  of  conductors.  2.  The  resistance  of  non- 
conductors or  insulators.  The  first  is  exemplified  in  the 
resistances  of  armatures,  coils  of  different  sorts,  rheo- 
stats, leads,  trolley  wires,  feeders,  etc. ;  the  second  in 
the  insulation  power  of  such  substances  as  mica,  paper, 
rubber,  glass,  oils,  air,  etc.  Conductors  are  either  solid 
or  liquid.  For  measuring  resistances  the  instruments 
used  are  those  already  described.  The  galvanometer,  or 
its  equivalent,  the  battery  and  the  standard  resistance, 
and  in  some  insulation  measurements  the  condenser  and 


MEASUREMENT    OF    RESISTANCE. 


'47 


electrometer.  All  measurements  are  based  on  methods 
dependent  directly  or  indirectly  upon  Ohm's  Law;  the 
conditions  being  so  arranged  that  the  unknown  resist- 
ance is  balanced  against  a  known  resistance. 

For  low  resistances,  the  method  of  the  *'  Comparison 
of  Potentials"  is  much  used,  and  it 
depends  upon  the  fact  that  when  an 
E.  M.  F.  is  applied  to  a  circuit  this 
E.  M.  F.  in  its  fall  from  the  positive 
to  the  negative  terminal  distributes 
itself  according  to  the  resistance  it 
meets;  where  there  is  the  greatest 
resistance,  the  greatest  P.  D.  exists; 
if  two  or  more  parts  of  a  circuit  are 
of  the  same  resistance  then  must 
the  P.  I),  across  them  be  the  same 
if  their  currents  are  the  same.  To  insure  that  the  cur- 
rent in  both  shall  be  the  same,  the  standard  resistance 
and  that  to  be  measured  are  placed  in  series  as  in  Fig.  46, 
where  B  is  the  source,  of  current,  R  a  standard  resist- 
ance, and  .r,  that  to  be  measured.  G,  is  a  high  resistance 
galvanometer,  S,  its  shunt,  and  A,  a  device  for  connect- 
ing G  to  read  the  P.  D.  across  R  or  ..r  at  will.  A  con- 
sists of  a  hard  wood  block  6"  X  4*,  with  six  3/4"  holes 
|]=nrj=r)  bored  half  through  its  face.  Let  in  from  the 
^  edge  and  attached  to  copper  lugs  on  the  bottom 
FIG.  47.  Qf  tnese  holes  are  Xo.  8  or  No.  10  soft  copper 
wires  provided  with  connectors  or  carried  to  binding  posts. 
The  holes  are  then  filled  three-quarters  full  of  mercury.  A 
rocking  block  is  used  for  cross-connecting  and  is  made  as 
follows:  Two  pieces  of  Xo.  10  wire  are  bent  as  shown  in 
Fig.  47,  with  the  middle  leg  longer  than  the  end  ones;  the 


148  TESTING    OF    DYNAMOS    AND    MOTORS. 

legs  are  at  such  a  distance  apart  as  to  fit  into  the  three 
sets  of  holes.  The  two  pieces  are  fastened  to  a  small 
block  so  that  they  move  together.  Since  the  middle 
legs  are  longest,  the  block  will  rock  upon  them,  and 
according  as  the  block  is  depressed  on  the  right  or 
left,  will  the  galvanometer  be  placed  across  x  or  ft 
respectively. 

The  test  consists  in  sending  the  same  current  through 
R  and  x  and  then  comparing  the  deflections  due  to  their 
respective  resistances.  Thus,  call  Dl  the  deflection  due 
to  R  and  Z>a  that  due  to  x. 

Then  Z>,    :   Z>2   ;  ;  R   :   x,  and 


x  -      * 
-— 

Now  since  Z>2  -=-  Dl  is  simply  a  ratio,  it  is  not  necessary 
to  know  the  actual  voltage  value  of  a  deflection,  but  it 
is  necessary  that  the  deflection  be  proportional  to  the 
voltage  applied:  /.  e.,  we  must  be  certain  that  if  i  volt 
causes  a  deflection  of  20  divisions,  2  volts  will  cause  a 
deflection  of  40.  The  method  is  a  reliable  one  and  is 
universally  used  for  measuring  fields,  armatures,  shunts, 
and  transformer  coils,  in  fact  any  low  resistance.  The 
galvanometer  must  be  a  reliable  one,  such  as  a  Thomson 
mirror  galvanometer,  and  R  must  be  an  accurate  standard. 
The  method  is  adapted  to  locating  crosses  and  open  cir- 
cuits in  armatures.  In  winding  and  connecting  armatures 
it  is  possible  that  a  section  may  get  a  turn  too  much  or  too 
little;  a  drop  of  solder  or  a  metal  filing  may  lie  across 
neighboring  commutator  bars,  or  turns  of  wire;  rough 
handling  of  insulation  may  have  resulted  in  the  crossing  of 
two  wires,  or  the  whole  armature  may  have  been  wound 


MEASUREMENT    OF    RESISTANCE. 


149 


an  inferior  quality  or  wrong  size  of  wire.  Under  any 
of  these  conditions,  or  those  of  an  open  circuit  or  loose 
connection,  the  galvanometer  deflection  will  show  dis- 
crepancies when  compared  with  those  taken  on  a  similar 
armature  known  to  be  sound. 

The  connections  for  the  test  are  shown  in  Fig.  48, 
where  A  is  the  armature,  B  the  source  of  current 
(usually  several  storage  cells),  R  the* 
standard  resistance,  r  a  variable  re- 
sistance to  assist  in  regulating  the 
current  7,  and  G  the  galvanometer. 
By  means  of  the  galvanometer  and 
standard  R  we  first  get,  by  varying 
/,  a  deflection  which  experience  has 
taught  us  is  convenient  to  compare 
with  that  caused  by  the  resistance 
of  A. 

On  two-pole  machines  the  service 

lines  are  held  to  opposite  commutator  bars  as  shown  in 
Fig.  48.  On  four-pole  machines  these  lines  must  include 
one-fourth  the  circumference  of  the  commutator,  since 
this  is  the  distance  from  the  -f-  to  -  -  brushes  when  the 
machine  is  running,  while  on  machines  of  n  poles  the 
portion  to  be  included  is 

i 
n 

of  the  circumference.  Care  must  be  taken  that  the  galva- 
nometer lines  make  good  contact  on  the  commutator. 
The  latter  must  be  free  from  dirt,  oil,  and  shellac,  and 
the  lines  had  best  make  contact  with  the  commutator 
itself  rather  than  with  the  brushes,  thus  preventing  the 


TESTING    OF    DYNAMOS    AND    MOTORS. 


resistance  of  brush  contact  raising  the  indicated  resist- 
ance of  the  armature.     In  practice  there  is  a  frame  so  con- 
structed as  to  hold  four  brushes,  two  for  the  main  current, 
and  two  smaller  ones  for  drop  lines.     To  throw  the  galva- 
nometer alternately  to  R  and 
A  we  may  use  either  a  three- 
way  switch  or  the  device  of 
•    Fig.  47.     Taking  the  drop  on 
J?  and  A  the    latter   can    be 
figured    as    above.     By    thus 
measuring  the  resistance  be- 
tween    opposite     bars     and 

knowing  what  the  resistance  should  be,  or  by  simply 
comparing  the  deflections  with  the  known  proper  deflec- 
tion, any  error  in  winding  or  insulation  or  loose  con- 
nection can  be  detected.  For  example,  in  Fig.  49 
suppose  an  open  circuit  to  exist  in  coil  No.  7.  That 
half  of  the  armature  will  carry  no  current  and  the  resist- 
ance between  terminals  being  doubled,  there  now  being 
but  one  path  from  brush  to  brush  instead  of  two,  the 
deflection  will  be  much  higher  than  it  ought  to  be.  If  the 
break  is  in  the  winding  itself  the  de- 
flection will  be  abnormal  all  round  the 
commutator;  but  if  in  one  of  the  con- 
necting wires  as  at  a  in  Fig.  50,  the 
needle  will  show  no  abnormal  deflec- 
tion except  when  this  bar  is  under  the 
brush,  for  at  all  other  positions  the 
break  has  no  influence.  Should  the 
the  wrong  number  of  turns  per  section,  or  the  wire  be  a 
little  hard  or  above  or  below  gauge,  the  fact  will  be  shown 
by  a  uniformly  "off"  deflection — too  high  if  the  effect 


FIG.  50. 
winder    get    on 


MEASUREMENT    OF    RESISTANCE.  151 

has  been  to  increase  the  armature  resistance,  too  low  if  it 
has  been  diminished. 

To  locate  a  faulty  section,  be  it  a  cross  in  the  coil  or 
commutator,  a  wrong  number  of  turns,  a  loose  connec- 
tion or  open  circuit,  the  test  is  somewhat  modified.  The 
main  current  enters  and  leaves  the  armature  at  the  same 
points  as  before,  but  the  'galvanometer  lines  touch  on 
adjacent  bars  and  therefore  include  but  a  single  section, 
and  the  drop  on  each  section  is  taken.  This  is  known 
as  the  bar  to  bar  test,  and  is  very  effective.  All  arma- 
tures should  be  subjected  to  this  test  before  the  binding 
wires  are  put  on,  as  frequently  the  fault  can  be  easily  re- 
moved and  a  burn-out  avoided.  In  passing  from  bar  to  bar 
it  will  soon  be  seen  what  the  normal  deflection  is  going 
to  be,  and  any  marked  deviation  indicates  a  defect.  A 
"high"  deflection  indicates  a  loose  connection  or  open 
circuit,  or  possibly  a  turn  too  much  in  the  section.  In 
case  of  open  circuit  the  galvanometer  will  show  no  deflec- 
tion until  its  lines  span  the  bars  which  include  the  break, 
that  is  if  the  break  is  in  the  winding  itself.  Where  the 
winding  itself  is  not  brought  down  to  the  commutator, 
but  is  continuous  and  has  leads  tapped  on,  the  galvanom- 
eter will  show  a  deflection  until  one  of  its  lines  rests  on 
the  bar  to  which  the  broken  lead  runs,  when  the  deflec- 
tion will  be  zero.  A  "low"  deflection  indicates  a  cross 
or  short  circuit  more  or  less  serious.  This,  we  have 
seen,  can  be  caused  by  filings,  solder,  or  wires  touching 
through  abraded  insulation. 

The  "bar  to  bar  "  test,  when  the  galvanometer  lines 
include  only  one  section,  is  open  to  the  objection  that 
it  will  not  detect  crosses  between  adjacent  sections, 
but  this  objection  can  be  eliminated  by  including  be- 


TESTING    OF    DYNAMOS    AND    MOTORS. 


tween  the  leads  two  sections  instead  of  one.  Other 
things  being  unaltered  the  normal  deflection  should  be 
twice  that  due  to  a  single  section.  If  in  any  case  it 
is  not,  the  sections  can  be  tested  individually;  if  each 
is  all  right  in  itself  the  trouble  must  be  between  them. 
The  test  can  be  further  modified  so  as  not  only  to 
apply  the  galvanometer  leads  to  adjacent  bars  but  the 
current  leads  also.  The  objection  to  this  arrangement 
is  that  on  ring,  or  other  armatures  where  the  connecting 
leads  are  tapped  on  to  the  main  wind- 
ing, the  test  would  not  reveal  a  broken 
lead  without  opening  the  main  circuit. 
On  armatures  wound  with  copper  bars 
whose  outer  surface  constitutes  the 
commutator,  a  fault  can  be  located  to 
within  an  inch  or  so.  The  method  is 
also  valuable  for  locating  grounds  in 
an  armature.  The  connections  are 
shown  in  Fig.  51.  As  in  the  measure- 
ments of  armature  resistance  the  current  enters  and  leaves 
at  the  same  points  as  when  the  armature  is  in  actual  ser- 
vice. The  machine  of  Fig.  51  is  a  bipolar,  so  that  the 
current  enters  and  leaves  at  opposite  points  of  the  com- 
mutator. B  is  the  source  of  current,  G  is  a  galvanometer, 
one  of  whose  terminals  is  attached  to  the  armature  shaft, 
the  other  being  free  to  move  along  from  bar  to  bar  on 
the  commutator.  Suppose  a  ground  to  exist  at  a;  i.  e., 
through  some  defect  in  insulation  the  wire  touches  the 
iron  core.  Ordinarily  the  battery  current  enters  the 
armature  at  M,  flows  around  the  two  halves  and  leaves  at 
TV7,  the  single  ground  at  a  having  no  effect  whatever. 
If  now  the  free  galvanometer  line  be  touched  to  any  part 


FIG.  51. 


MEASUREMENT    OF    RESISTANCE.  153 

of  the  commutator  the  latter  becomes  grounded  in  two 
places  — through  the  defect  and  through  the  galvanom- 
eter. .  The  galvanometer  now,  being  a  shunt  to  part  of 
the  armature,  carries  current,  and  indicates  the  fact  by  a 
deflection.  The  galvanometer  really  gets  the  drop  on 
the  armature  wire  included  between  the  fault  and  the 
galvanometer  lead  which  touches  the  commutator.  Now 
if  the  free  galvanometer  lead  be  moved  around  the  com- 
mutator it  brings  the  two  grounds  nearer  together,  so 
that  the  galvanometer  gets  less  drop,  and  the  deflection 
grows  less  and  less,  until  when  the  lead  reaches  a,  both 
galvanometer  leads  are  at  the  same  potential,  and  the 
deflection  becomes  o.  When  the  free  lead,  ^,  passes  a 
the  deflection  rises  again  but  with  reversed  direction.* 
This  is  taken  advantage  of  where  the  galvanometer  is 
not  very  sensitive,  for  then  equal  deflections  can  be 
gotten  on  opposite  sides  of  o,  and  the  fault  located 
halfway  between  these  points.  Having  thus  located  the 
ground  the  current  brushes  M  N  should  be  shifted  and 
the  ground  again  located.  The  two  determinations 
should  coincide.  This  precaution  is  necessary,  because 
between  M  and  N  there  are  two  paths  and  the 
drop  of  potential  through  both  is  the  same.  For  every 
point  in  path  M  a  N  there  is  a  point  in  Me  N  at  the 
same  potential;  so  there  is  a  point  c  corresponding  to 
point  a,  where  the  fault  exists,  and  this  point  c  will  give  a 
zero  deflection  also.  When  M  and  N  are  shifted  the 
point  c  shifts  but  a  does  not.  There  will  be  a  new  point 
at  the  same  potential  as  a,  but  a  being  common  to  both 
tests  is  the  point  sought. 

A  second  method  of  measuring  moderate  resistances  is 
called  the  "  Vienna  Method,"  and  is  especially  applicable 


154  TESTING    OF    DYNAMOS    AND    MOTORS. 

where  the  portion  of  circuit  to  be  measured  is  in  service, 
as  in  the  case  of  burning  lamps  or   fields    on  running 

machines.     Fig.  52  gives  con- 

— 1||||| 1  A  I — ^— r- 1     nections      for      measuring     a 

burning  lamp.  The  source 
of  current,  B,  ammeter,  A, 
and  lamp,  Z,  are  in  series, 


Lf 

FlG  while  voltmeter,  V,  reads  the 

potential  difference  across  the 

lamp.     If   7  be   the  current  and  E  the   P.  D.  then   by 
Ohm's  law  the  resistance  is 


.. 

Strictly  speaking  there  is  an  error  due  to  the  fact  that 
V's  presence  in  shunt  with  Z,  lowers  the  resistance 
between  the  lamp's  terminals,  for  the  resistance  of  Z 
and  Fin  multiple  must  be  lower  than  that  of  Z  alone, 
and  hence  the  P.  D.  across  the  two  is  less  than  it  would 
be  across  Z.  On  the  other  hand,  decreasing  the  circuit 
resistance  increases  Z,  so,  on  the  whole,  considering  the 
law, 


we  see  that  decreasing  E  and  increasing  Z  lowers  the 
indicated  value  of  Z's  resistance.  In  all  but  most  par- 
ticular work  the  results  obtained  are  satisfactory,  for 
since  the  meter  resistance  is  from  15,000  to  75,000  ohms 
it  shunts  but  little  current  from  the  lamp  resistance  of 
250  ohms.  If  it  is  desirable  to  allow  for  this,  the  correc- 
tion can  be  made  in  either  of  two  ways.  F's  current  can 
be  calculated  from  K's  resistance  and  indicated  voltage, 


MEASUREMENT    OF    RESISTANCE.  155 

and  this  value  subtracted  from  the  amperemeter  to 
obtain  the  true  value  of  /.  E  divided  by  this  cor- 
rected value  of  /  gives  Z's  re-  B 
sistance.  Or  the  connections 
may  be  changed  to  those  of 
Fig-  53>  where  V  reads  the 
voltage  across  both  A  and  Z, 
while  A  reads  only  Z's  current. 
Knowing  A's  resistance  and  cur- 
rent the  drop  through  it  (/  ;-A)  can  be  figured  and  sub- 
tracted from  F's  indication  to  get  the  true  drop  across  Z. 
The  choice  between  the  connections  of  Figs.  52  and  53 
depends  upon  how  accurately  A's  and  F's  resistances  are 
known.  Absolute  freedom  from  these  corrections  can  be 
secured  by  using  an  eleetrometer  for  reading  the  voltage. 
The  electrometer's  action  depends  upon  the  fact  that  two 
objects  which  are  statically  charged  at  different  poten- 
tials, attract  each  other:  if  one  is  free  to  move  it  will  do 
so.  An  electrometer  has  two  metal  plates  opposite  each 
other,  one  of  which  carries  an  indicator  and  is  free  to 
move.  One  plate  is  attached  to  the  one  point,  the  other 
plate  to  the  other,  of  the  two  points,  between  which  the 
P.  D.  is  to  be  measured.  By  means  of  a  voltmeter  and  its 
multiplier  the  electrometer  is  calibrated;  thereafter  it  can 
be  used  to  'measure  voltage  and  has  the  advantage  that 
since  no  current  passes  through  it  does  not  disturb  the 
conditions  under  which  it  is  desired  to  be  used.  They  are, 
however,  only  used  for  measuring  very  high  voltages. 
The  main  advantage  of  the  foregoing  method  over  that 
of  comparison  of  potentials  is  that  no  standard  shunt  is 
required.  On  the  other  hand  the  comparison  method 
does  not  require  a  galvanometer  calibrated  to  read  volt- 


'56 


TESTING    OF    DYNAMOS    AND    MOTORS. 


eoo 


FIG.  54. 


age.  The  choice  between  the  two  would  then  depend 
upon  what  standard  is  available.  The  following-  method 
is  an  adaptation  of  the  above  and  requires  no  calculat- 
ing. It  is  useful  in  checking  up 
resistances  that  are  almost  certain 
to  be  right,  or  in  measuring  where 
some  margin  is  allowed  on  both  sides 
of  a  given  value.  In  Fig.  54  A  and 
B  are  the  -j-  and  —  terminals  re- 
spectively of,  say,  a  500  volt  circuit, 
R  is  a  standard  resistance  across 
which  V  reads  the  drop,  and  at  r  is 
placed  the  resistance  to  be  tested. 
The  first  step  is  to  place  at  r  a  re- 
sistance known  to  be  right;  next  adjust  the  voltage  be- 
tween A  and  B  at  500:  close  the  circuit  and  note  F's 
deflection;  (call  it  200).  Any  time  thereafter  if  we  adjust 
E  to  500  and  get  200  across  R  we  may  be  certain  that  r 
is  what  it  ought  to  be.  The  advantage  of  putting  V 
across  R  instead  of  r  is  that  in  most  cases  it  saves 
changing  the  volt  lines. 

The  next  resistance  measuring  method  to  be  considered 
is  the  "  Bridge  Method."  The 
bridge  is  an  instrument  invalu- 
able to  an  electrical  workshop, 
testing  room,  laboratory,  or  line- 
man's "  shanty."  It  has  a  range 
unexcelled  by  any  other  resist- 
ance measuring  device.  Refer- 
ence to  Fig.  55  will  aid  in 
understanding  the  underlying  principle.  C  is  a  cell; 
G,  a  galvanometer,  none  of  whose  constants  need  be 


MEASUREMENT    OF    RESISTANCE.  157 

known,  since  it  is  to  simply  indicate  the  presence  of 
current  without  measuring  it.  One  galvanometer  ter- 
minal is  fastened  at  JV,  the  other  is  free  to  move. 
From  A  to  B  the  current  has  two  paths — A  N  B, 
and  A  M  B;  the  drop  of  potential  between  A  and 
/>,  being  a  definite  quantity,  is  necessarily  the  same  by 
both  paths.  For  every  point  in  A  N  B,  then,  there  is 
in  A  Af  B  a  point  at  the  same  potential,  and  a  galvanom- 
eter joining  these  points  will  give  no  deflection,  for  no 
current  will  flow  between  points  of  the  same  potential. 
Under  this  condition  we  have  what  is  called  a  balance, 
and  rl  :  ry  ;  ;  r  :  .-4  where  /-,,  ra,  ;-.,,  and  ;-4  are  respectively 
the  resistances  of  A  JV,  B  A7,  A  M,  and  B  M.  Knowing 
any  three  of  these  quantities  the  fourth  can  be  found.  To 
see  this  more  clearly,  suppose  the  two  paths  to  be  com- 
posed of  the  same  size  wire  and  that  N  has  been  secured 
at  the  middle  point  of  A  N  B.  Then,  if  the  wire  is 
assumed  to  be  of  uniform  cross-section,  the  only  point  to 
balance  Arwill  be  J/,  the  middle  point  of  A  MB.  Under 
this  condition  of  balance  A  N  =  B  N  and  A  M  =  B  M; 
therefore 

AN  _ 

and  also 

A  M 
TTM~ 

whence 

B~N  ~  ~B~M*  ° 

What  we  have  proven  to  be  true  in  this  special  case  is  true 
in  all  cases.  Again,  if  t\  be  the  current  in  A  N  B,  and 


158  TESTING    OF    DYNAMOS    AND    MOTORS. 

/2  that  in  A  MB,  the  P.  D.  between  A  and  B  is,  by 
Ohm's  law,  t\  rl  -)-  t\  r2 ;  also  it  is  /,  rb  -j-  /2  r4,  hence 
t\  t\  -j-  *,  ^2  —  *a  *"g  +  *2  r4-  Now  at  balance,  the  drop 
from  ^4  to  N  is  the  same  as  that  from  A  to  M ' ;  also 
drop  j?  N  =  'drop  B  M,  or  i\  r}  =  iz  rs  and  il  r2  —  /2  r4; 
whence  from  proportion  we  have  /2  :  ^  ;  ;  rl  :  rs  and 
*a  :  zi  •  •  ra  :  r4>  therefore  ^  :  ra  ;  ;  rs  :  r4.  And  this  is  the 
entire  theory  of  the  bridge.  If  we  put  rl  =  a;  r2  =  b; 
r3  —  r\  r4  =  x  (i.  <?.,  just  use  letters  that  are  most  popu- 
lar) we  have  a  :  b  \  \  r  :  x,  or 


whence  it  can  be  seen  that  it  is  not  necessary  to  know 
the  absolute  value  of  a's  and  <£'s  resistance,  but  only  their 
ratio;  and  this  multiplied  by  r  gives  x. 

In  all  forms  of  bridges  the  arms  a  and  b  are  called 
proportion  arms.  In  some  bridges  r  remains  constant 
while  the  ratio 

b_ 

a 

is  varied,  but  in  most  bridges  r  alone  is  varied. 

The  first  type  is  known  as  the  "  slide  wire 
bridge  "  and  is  shown  in  Fig.  56.  Between  two  heavy 
copper  bars  A  B  a  German-silver  wire  of  uniform 
cross-section  is  stretched  and  its  terminals  soldered. 
Under  the  wire  a  metre  rod  or  scale  is  laid,  carefully 
graduated  into  1,000  divisions.  If  the  wire  is  of  uniform 
cross-section  its  resistance  need  not  be  known,  otherwise 
it  must  be  calibrated  in  ohms  throughout  its  length. 
Suitable  terminals  are  provided  to  receive  the  known 


MEASUREMENT    OF    RESISTANCE. 


*59 


resistance,  ^t*,  and  the  unknown,  X.  With  connections  as 
in  the  figure,  M  is  moved  along  A  B  till  a  balance  obtains. 
Then  calling  the  resistance  of  A  M  —  a  and  B  M  —  b, 
we  have  a.  :  b  \  \  r  :  A',  and 


; . 


If  the  wire  A  B  is  of  uniform  cross-section,  we  can,  in- 
stead of  using  A  M's  and  B  J/'s  resistances,  use  their 
lengths,  which  are  in  the 
same  proportion,  whence 
R  is  the  only  resistance 
which  need  be  known. 
If  R  =  X  balance  will 
obtain  when  M  rests  at 
500  (t.  *•.,  in  the  middle 
of  the  scale).  Accord- 
ing as  R  is  greater  or 


1 

M 

JB 

Hill  

FIG.  56. 


less  than  X  the  balance  potjU  will  be  on  one  side  or  the 
other  of  the  500  mark.  Let  R  =  2  ohms  and  A'  =  3 
ohms :  then  balance  will  be  when  M  is  at  such  a  point  that 


a       2  ' 


/.  e.,  A  B  —  1,000  divisions  must  be  divided  into  two 
parts  which  shall  be  to  each  other  as  3  :  2,  therefore 
a  =  400  and  b  =  600.  Now  suppose  R  =  5  ohms  and 
suppose  a  balance  is  gotten  when  a  =  413  and  b  =  587; 
then, 


X  =-  x  JR  = 
a 


587 


X  5  = 


i6o 


TESTING    OF    DYNAMOS    AND    MOTORS. 


whence  X  —  7.  i  ohms.  One  must  understand  clearly 
which  length  is  a  and  which  b.  The  range  of  accurate 

measurement  depends 
upon  R's  value  ;  the 
accuracy  is  greatest 
x  when  R  =  X,  and  the 
probable  error  in- 
creases as  M  recedes 
from  the  middle  of  the 
scale.  In  using  such 
a  bridge  care  must  be 
taken  that  the  slid- 
ing contact  neither 

scrapes  nor  nicks  the  wire,  thereby  reducing  its  cross- 
section.  The  bridge  can  be  bought  for  less  than  ten 
dollars  and  made  for  about  half  this  sum. 

In  the  second  class  of  bridge  the  arms  a  and  ^,  known 
as  proportion  arms,  are  first  selected  to  suit  conditions, 
and  remain  of  fixed  value,  while  R  is  varied  to  get  a 
balance.  Fig.  57  illustrates  a  commercial  form  of  such 
a  bridge.  Its  accuracy  like  that  of  all  bridges  depends 
upon  the  ratio 

b 


FIG.  57. 


used,  upon  the  voltage  of  the  cells,  the  sensitiveness  of 
the  galvanometer,  and  upon  the  care  with  which  the 
instrument  is  kept  and  used.  A  B  and  B  C  are  the 
proportion  arms,  and  consist  of  three  coils,  of  10, 
100,  1,000  ohms  respectively.  By  means  of  these  coils 
we  can  employ  for 

b 


MEASUREMENT    OF    RESISTANCE.  l6l 

the  ratios 

I  I  I  10        100 

-  »-    >        -  or  — ,  - 

L          10      IOO  I  I 

according  as  A"  is  large  or  small,  if  we  use  the  same  coil 
of  both  arms,  then 

b        i 

a         i 

and  at  balance  R  —  X;  nor  can  we  measure  a  resistance 
exceeding  that  in  J?,  if  equal  arms  are  used. 

If*   :      JL   A' :=-*-;       if  sil,A'=io*; 

a  10  10  a  i 

,  b  i  R  .cb          100 

if  -  =  -  --,  A  =  -      ;  also  if  —  =   -       ,  A  =  100  R. 
a         100  100  a  i 

From  which  we  see  that  with  equal  arms,  the  largest 
resistance  possible  to  be  measured  is  the  total  resistance 
of  R\  the  smallest  is  that  of  the  lowest  coil  in  R.  With 
the  ratio  1/100  the  largest  measurable  value  of 

A"  =  Total  R  X  ioo, 

while  X  can  be  as  low  as  T/IOO  part  of  the  lowest  coil  in 
R,  and  so  on.  To  facilitate  fine  adjustments  of  R  re- 
sistances from  i  ohm  to  the  full  capacity  of  the  box  can 
be  introduced  by  withdrawing  those  plugs  which  short 
circuit  the  coils  required.  The  range  of  measurement  is 
from  i/ioo  of  the  value  of  the  lowest  coil  to  ioo  times 
the  total  value  of  R.  Suppose  the  lowest  coil  to  be 
.1  ohm  and  the  total  R  to  be  11,110  ohms.  The  range 
is  then  .001  ohm  to  i,  111,000  ohms,  or  over  a  megohm. 


162 


TESTING    OF    DYNAMOS    AND    MOTORS. 


There  are  provided  two  keys  for  the  battery  and  galva- 
nometer circuits  respectively.  In  many  boxes  the  keys 
are  placed  one  over  the  other,  so  that  a  single  pres- 
sure closes  first  the  battery  circuit,  then  that  of  the 
galvanometer.  The  object  of  closing  the  battery  circuit 


FIG.  58. 

first  is  to  allow  extra  currents  due  to  self-induction 
to  die  away,  and  leave  everything  in  a  state  of  equili- 
brium. If  the  X  arm  be  a  field  winding  or  other  coil 
having  self-induction  the  current  will  not  work  its  way 
through  this  arm  as  quickly  as  it  will  through  the  others, 
with  the  consequence  that  the  needle  deflects  upon  clos- 
ing the  key,  even  though  there  may  be  a  perfect  balance 
so  far  as  concerns  ohmic  resistance.  By  closing  the 
battery  key  first,  and  allowing  the  current  to  reach  a 
steady  value,  this  objectionable  feature  is  eliminated. 
Figs.  58,  59,  and  60,  give  the  perspective  view,  plan,  and 
connections  of  the  latest  form  of  portable  bridge  gotten 
out  by  Queen  &  Co.  The  instrument  is  of  a  high  class 
of  workmanship,  is  convenient,  accurate  and  merits  the 


MEASUREMENT    OF    RESISTANCE. 


163 


reputation  it  has  won.  Its  theoretical  range  is  from  .001 
ohm  to  11,110,000  ohms,  but  for  the  higher  resistances 
higher  external  voltages  must  be  used.  One  feature  which 
commends  the  bridge  to  commercial  testing  room  use  is 
its  dead  beat  galvanometer.  Readings  can  be  quickly 
taken  and  are  not  influenced  by  external  magnetic  fields. 
Mounted  in  a  handsome  mahogony  box,  8*^"  x  524*  X  5", 
and  weighing  but  6  pounds,  it  forms  an  invaluable 
addition  to  a  testing  set.  Fig.  59  gives  the  connection 
scheme  and  Fig.  60  the  application  of  this  scheme  to  the 


FIG.  59. 


FIG.  60. 


bridge  proper.  Let  A  be  the  left  bridge,  arm;  B,  right 
arm;  R,  resistance  in  rheostat  arm;  A',  resistance  to  be 
measured.  When  A  plugs  ^and  X  plugs  B, 


When  A  plugs  X  and  R  plugs  B, 

X         A  A 

-  =  -,orX  =  -JX. 

To  measure  resistance,  first  connect  the  terminals  of  the 
resistance  to  be   measured  to  the  two  binding  posts  seen 


164  TESTING    OF    DYNAMOS    AND    MOTORS. 

at  the  lower  left  hand  corner  of  the  set,  Fig.  58,  and 
make  an  estimate  of  X's  approximate  value;  suppose  it 
to  be  in  the  neighborhood  of  400  ohms;  plug  A  to  R  and 
X  to  B  and  unplug  the  100  ohm  coil  in  both  A  and  B, 
also  the  500  ohm  coil  in  R.  Put  on  one  cell  of  the 
battery  and  close  the  key  for  an  instant.  (It  is  well  to 
start  with  small  battery  power  so  that  no  harm  results 
in  case  X's  estimated  value  is  far  from  correct.)  If  the 
needle  swings  to  -)-,  R  is  too  high  ;  if  to  —  ,  R  is  too  low. 
By  altering  R,  and  if  necessary  increasing  the  battery 
power,  a  balance  can  be  effected  and  X  determined  to 
within  i  ohm.  If  it  is  desired  to  be  more  accurate 
make  A  i  ohm  and  B  10  ohms,  so  that 


R  must  then  be  made  about  10  times  the  approximate 
value  of  X,  Upon  securing  a  balance  the  error  will  be 
but  i/io  as  great  as  in  the  first  adjustment.  Still  greate*- 
accuracy  can  be  gotten  by  using  the  ratio  1/100,  and  a 
value  of  R  about  100  times  the  estimated  value  of  X.  If 
the  resistances  to  be  measured  are  higher  than  the  total 
resistance  of  the  rheostat,  plug  A  to  X  and  B  to  R. 
Suppose  X  is  less  than  100  times  the  total  resistance  of 
R,  say  880,000  ohms:  unplug  TO,  in  A,  1,000  in  J3,  and 
8,800  in  R  ;  balance  and  proceed  as  in  the  two  cases 
above. 

To  use  the  set  as  an  ordinary  resistance  box  without  the 
galvanometer  and  battery,  arrange  the  reversing  blocks, 
A,  R)  B,  X,  as  in  preceding  cases  and  make  connections 
to  the  binding  posts.  It  is  immaterial  upon  which 
diagonal  the  plugs  in  the  reversing  blocks  lie.  To 


THOMSON'S 
SLIDE    BRIDGE 

FOR  MEASURING  VERY 
LOW  RESISTANCES 


EXAMPLE: 
R  =.000322 

a     1000 
6-=ioo 

X=. 000322  X  .1 
=.0000322  OHMS 
NOTE: 

SEE  MUNROE  A  JAMIE8ON, 
7TH  ED.  PAGE  08 


FlG.    62. 


l66  TESTING    OF    DYNAMOS    AND    MOTORS. 

exclude  bridge  arms  and  use  rheostat  only,  connect  A  or 
B  to  R  and  X. 

To  measure  very  low  resistances  such  as  are  met  with 
in  cable  work,  the  error  due  to  contacts  must  be  elimi- 
nated, so  that  the  usual  form  of  bridge  is  by  no  means 
satisfactory.  For  this  work  Sir  William  Thomson  has 
devised  a  form  of  Wheatstone  bridge  in  which  this  error 
does  not  occur.  Fig.  61  illustrates  the  principle  while 
Fig.  62  shows  the  connections  for  the  bridge  itself.  B 
is  a  battery,  and  G  a  galvanometer  of  great  sensitive- 
ness; R,  a  standard  resistance,  and  X,  that  to  be 
measured,  are  soldered  together.  Resistances  a,  b,  c,  and 
</are  so  great  as  to  render  contact  resistance  negligible. 
K  is  a  key  for  closing  J?'s  circuit  and  K'  a  key  for  clos- 
ing G's.  The  condition  of  balance  as  indicated  by  the 
galvanometer,  is  a  i  b\\c  I  d  \\  JR  '.  X\  or 

x=LR. 

a 

This  can  be  shown  as  follows  :  suppose  R  and  X  to  be 
absent;  S'  coincides  with  61  and  T'  with  T,  and  we  have 
an  ordinary  Wheatstone  bridge  where  a  :  b  \  \  c  :  d  or 
a  d  =  be.  Now  introduce  R  and  X,  varying  R  till  a  bal- 
ance obtains ;  then,  a  :  b\  \  R  +  c  :  X  -\-  d,  or  a  X  -j-  a  d  — 
b  R  +  b  c;  but  b  c  —  a  d.  Therefore  a  X  =  b  R>  and 

Y         b    /? 
X.  =  —  jft 

a 

as  above.  The  manner  of  varying  R  is  shown  in  Fig.  62, 
and  is  done  very  gradually  by  means  of  a  lever.  The 
lettering  is  similar  to  that  of  Fig.  61.  The  manner  of 
using  this  bridge  does  not  differ  from  that  of  the  ordinary 


MEASUREMENT    OF    RESISTANCE.  167 

type.  Its  accuracy  depends  upon  the  accuracy,  prima- 
rily of  7?,  and  also  of  a  and  b.  An  example  will  help  us. 
Let  a  —•  1,000,  b  =  100,  R  =  .000322;  then 

b  100 

X  =  R  -  =  .000322  -      -  =  .0000322. 
a  1,000 

The  Wheatstone  bridge  is  by  far  the  most  universally 
used  instrument,  and  its  thorough  understanding  is  of 
indispensable  value.  We  therefore  feel  justified  in  adding 
the  following  general  remarks 
incidental  to  bridge  practice.  In 
using  any  bridge  the  first  step  to 
take  is  to  insure  that  all  connec- 
tions are  correct  and  that  all 
circuits  are  intact.  With  equal 
resistances  in  the  proportion 

arms  take  out  the  10  ohm   plug 

FIG.  63. 
in  the   rheostat:     holding  apart 

the  test  lines  going  to  X,  press  successively  the  battery 
and  galvanometer  circuit  keys.  In  the  typical  diagram 
of  Fig.  63,  A  and  B  are  the  proportion  arms,  R  the 
rheostat,  and  X  the  unknown  resistance.  When  the  test 
lines  leading  to  X  are  held  apart,  the  X  arm  is  open  cir- 
cuited, its  resistance  is  infinite,  and  current  from  arm  B 
therefore  flows  across  through  the  galvanometer  causing 
a  decided  deflection,  due  to  the  excess  of  resistance  of  X 
over^;  for  X  equals  oo  while  R  equals  10.  Say  the  de- 
flection is  to  the  right.  Now  short  circuit  the  test  lines 
by  holding  them  together:  upon  pressing  the  keys  there 
will  be  a  deflection  to  the  left,  for  R  now  equals  10 
while  X,  being  short  circuited,  equals  o.  If  both  deflec- 
tions are  in  the  same  direction  the  indications  are  that 


1 68  TESTING    OF    DYNAMOS    AND    MOTORS. 

there  is  either  an  open  or  a  short  circuit  in  the  test  lines. 
If  the  line  is  broken  holding  the  lines  apart  will  not  alter 
conditions,  nor  will  crossing  them  if  they  are  already 
crossed  elsewhere.  It  is  barely  possible  that  the  bridge 
may  have  some  internal  disorder.  An  open  or  short  cir- 
cuit in  A  will  give  the  same  symptoms  as  if  in  X,  so  also 
with  B  and  R.  But  such  a  trouble  can  be  quickly  located 
by  measuring  the  same  resistance  with  different  ratios. 
It  is  possible  one  line  wire  may  be  broken  inside  the 
insulation  or  the  two  wires  may  be  mashed  together. 
Under  either  condition  it  will  be  impossible  to  get  a 
balance;  for  in  one  case  there  is  not  enough  resistance  in 
R  to  balance  when  X  equals  oo,  and  in  the  second  case, 
none  small  enough  to  balance  when  X  equals  o.  If  while 
securing  a  balance  the  needle  give  a  sudden  lurch  for  some 
unaccountable  reason,  it  is  probably  due  to  a  loose  con- 
nection. If  successive  measurements  of  the  same  resist- 
ances show  discrepancies  in  results,  it  is  likely  due 
either  to  contact  troubles  or  to  the  presence  of  dust 
between  the  rheostat  blocks.  A  common  source  of 
annoyance  is  in  screwing  the  plugs  in  just  tight  enough 
to  trap  a  layer  of  dust  in  between  them  and  the 
blocks.  Failure  to  get  any  deflection  on  the  galvanom- 
eter may  be  due  to  open  circuit  either  in  its  own  or 
the  battery  circuit.  In  the  first  place  a  battery  lead 
may  have  become  corroded  or  the  contact  tips  of  K  or 
K'  may  be  oxidized  and  refuse  to  carry  current;  care- 
ful inspection  will  generally  reveal  troubles  of  this 
nature  :  but  if  it  is  necessary  to  test  out,  first  disconnect 
the  galvanometer  and  replace  it  by  a  piece  of  wire;  do 
the  same  with  the  battery.  This  leaves  the  circuits  as 
complete  as  before  and  we  have  a  galvanometer  and 


MEASUREMENT    OF    RESISTANCE.  169 

battery  to  test  with.  J<;in  one  side  of  G  to  one  side  of  the 
battery.  From  the  two  remaining  terminals  bring  two 
small  w-ires  to  serve  as  test  lines.  Touch  them  together; 
ifcV  deflects,  G  and  Care  all  right.  If  G  does  not  deflect 
it  may  be  due  to  a  poor  contact  somewhere;  be  certain 
that  this  is  not  so.  There  may  be  a  loose  connection 
inside  the  galvanometer.  If  there  are  two  cells  of 
battery  they  may  be  joined  in  opposition.  If  only  one 
cell,  its  plates  may  be  touching  inside.  The  needle  if 
suspended  may  have  a  broken  fibre,  if  pivoted,  may  be 
stuck:  the  ingoing  and  return  galvanometer  wires  may 
be  crossed  so  as  to  cut  out  the  coil.  Once  assured  that 
the  test  circuit  is  all  right  test  across  the  bridge  keys;  G 
should  deflect,  showing  the  circuits  to  be  all  right.  If  it 
does  not  a  metallic  wedge  can  be  shoved  into  the  key  of 
the  defective  circuit  so  as  to  complete  it  there.  The 
test  lines  are  then  made  to  span  different  parts  of  the 
circuit  till  G  deflects,  and  we  know  that  the  break  is 
repaired. 

We  have  seen  that  with  a  ratio 

I  10  100 

—  or  —  or  — 
i         10         100 

no  greater  resistance  can  be  measured  than  is  contained 
in  ft.  When  using  this  ratio,  the  resistance  can  be  got- 
ten only  to  a  first  approximation  unless  it  happens  to  be 
an  exact  number  of  ohms  within  the  range  of  R.  More 
frequently  a  point  is  reached  where  the  insertion  of  i 
ohm  in  R  causes  a  deflection  one  way,  and  its  withdrawal 
a  deflection  the  other,  showing  the  value  of  X  to  lie 
between  the  two  values  of  R. 

Suppose  9  and  10  ohms  cause  deflections  respectively 


I7O  TESTING    OF    DYNAMOS    AND    MOTORS. 

to  right  and  left:  if  the  two  deflections  are  of  the  same 
magnitude  it  indicates  that  X  lies  midway  between  9  and 
10  ohms,  and  according  as  one  or  the  other  resistance 
gives  the  greater  deflection  the  value  of  X  will  be  the 
more  removed  from  its  value.  To  obtain  a  closer  approx- 
imation the  ratio 

10         100 
-or_, 

in  some  cases,  can  be  used.  The  larger  the  current 
around  the  coils  of  any  galvanometer  of  given  sensibility 
the  larger  will  be  its  deflection.  In  measuring  high 
resistances,  on  a  bridge,  in  order  to  increase  the  deflec- 
tion for  a  given  difference  between  R  and  X,  a  higher 
voltage  is  used.  As  long  as  high  resistances  are  being 
measured,  that  necessary  to  balance  them  is  also  high, 
and  each  proportion  arm  having  a  high  resistance  in 
series  with  it  is  less  liable  to  damage  from  the  increased 
voltage:  but  when  measuring  low  resistances  the  i 
ohm  or  10  ohm  coil  of  the  proportion  arm  is  liable  to 
injury,  and  the  voltage  must  be  lowered  to  suit  the  new 
conditions. 

On  some  bridges  the  plugs  are  drawn  out  to  introduce 
resistance  while  in  others  they  must  be  inserted.  In 
the  first  case  every  coil  is  short  circuited  by  a  plug  which 
must  be  drawn  out  to  let  the  coil  into  circuit.  In  the 
second  case  it  is  only  by  inserting  a  plug  that  the  circuit 
through  the  coil  is  completed  as  seen  in  Fig.  65.  The  coil 
in  this  case  is  continuous,  and  is  tapped  off  at  intervals  to 
plugs  which  are  to  be  plugged  to  a  bar  running  the  full 
length  of  the  coil.  The  latter  has  the  advantage  that 
fewer  plugs  are  used,  and  hence  there  are  fewer  contacts 


MEASUREMENT    OF    RESISTANCE.  17 1 

to  introduce  error.  On  many  bridges  the  proportion 
arms  are  identical,  but  on  others  one  arm  may  have 
i,  10,  100  ohm  coils,  and  the  other  10,  100,  1,000  ohm  coils. 
According  as  one  ratio  or  another  is  used  the  error  intro- 
duced in  results  by  an  error  of  i  ohm  in  the  adjustment 
of  7?,  varies.  Using  a  ratio 


i 

an  error  of  i  ohm  in  fi  causes  an  error  of  i  ohm  in  A". 
That  is,  if  through  lack  of  care  in  adjustment  a  balance  is 
taken  at  9  when  it  really  calls  for  10,  A''s  value  will  be  just  i 


FIG.  64.  FIG.  65. 

ohm  out.  Such  an  error  can  be  caused  by  undue  haste  in 
adjusting  7?,  by  dust  in  the  rheostat,  or  by  not  having  the 
needle  at  the  zero  of  torsion.  Some  suspended  galvanom- 
eter needles  intended  for  bridge  work  have  their  side  play 
limited  by  two  non-magnetic  stops  placed  on  both  sides  of 
the  needle,  and  the  normal  position  of  the  needle  is  midway 
between  these  stops.  If  the  directive  force  of  the  magnetic 
field  holds  the  needle  over  against  one  of  these  stops,  and 
if  instead  of  restoring  the  needle  to  its  proper  position  by 
turning  the  instrument  it  be  done  by  turning  the  torsion 
head,  the  needle  then  holds  its  position  against  the 
twisting  moment  of  the  fibre.  Under  such  a  restraining 
force  the  sensibility  of  the  needle  is  much  diminished. 


172  TESTING    OF    DYNAMOS    AND    MOTORS. 

The  writers  have  preferred  to  do  away  with  the  stops  on 
either  side  the  needle,  to  allow  it  to  find  its  own  position  of 
rest  each  time,  and  then  to  give  it  a  directive  force  about 
this  point  by  means  of  a  magnet  laid  somewhere  near  it. 
Using  the  ratio 

10 
i 

an  error  of  i  ohm  in  R  causes  a  difference  of  .1  ohm  in 
X\  with  the  ratio 

100 

i 

the  error  in  X  is  .01  ohm,  and  with  the  ratio 

1,000 
i 

.001  ohm.     With  a  ratio 

10 
i 

suppose  a  balance  to  have  been  gotten  with  R  =  25. 
Then,  10  :  i  ;  \  25  :  X,  and  X  =  2.5  ohms.  Should  the 
balance  properly  have  been  at  24,  corresponding  to  a 
resistance  of  2.4  ohms,  the  error  would  be  2.5  —  2.4  =  .  i 
ohm.  To  measure  greater  resistances  than  that  in  R, 
the  ratios 

10         10          i 


100       1,000       I,OOO 

are  used.  Results  are  not  as  accurate  as  when  using  the 
inverse  of  these  ratios,  for  in  the  first  case  an  error  of 
i  ohm  in  R  means  an  error  of  10  ohms  in  X.  In  the  last 
case  a  difference  of  i  in  R  causes  a  difference  of  1,000 
in  JVT<  These  ratios  are  used  mainly  for  measuring  resist- 
ance of  insulation  where  the  error  may  be  very  large,  and 
still  be  but  a  fractional  per  cent,  of  the  total  resistance. 


MEASUREMENT    OF    RESISTANCE.  173 

In  all  low  resistance  work  the  resistance  of  the  test 
lines  must  be  taken  into  consideration,  and  in  some  work 
that  of  the  bridge  rheostat  arm.  To  measure  the  test 
lines,  clamp  their  further  ends  together  and  if  possible 
get  a  balance  with  the  ratio  1,000  to  10.  Suppose  this 
to  be  impossible  and  that  the  insertion  or  withdrawal  of 
i  ohm  causes  respectively  a  -j-  and  —  deflection.  Sup- 
pose the  galvanometer  dial  to  be  graduated  and  that 
with  R  =.  90  a  -f-  deflection  of  27  obtains,  and  that 
for  R  —  91  a  —  deflection  of  17.  A  balance  at  R  —  90 
would  correspond  to  A'  =  .9  ohm:  and  a  balance  at 
R  =  91  to  A"  =  .91  ohm,  a  difference  of  .01  ohm. 
This  difference  of  .01  ohm  causes  a  deflection  of  27  -f 
17  =  44  divisions:  one  division  would  be  caused  by 

.01 

=  .000227  ohm. 
44 

Since  90  ohms  causes  a  deflection  of  -f  27  such  a  resist- 
ance must  be  added  to  90  as  will  bring  the  needle 
back  27  divisions  to  zero,  corresponding  to  a  balance; 
.000227  ohm  will  move  the  needle  i  division;  to  move 
it  back  27  divisions,  .000227  x  27  =  .006129  ohm  must 
be  added  to  90,  making  A",  at  the  balance  point,  = 
.90006.  This  method  of  ascertaining  the  balance  point 
without  actually  effecting  it,  is  known  as  the  method 
of  interpolation.  To  enable  finer  measurements  to 
be  actually  made  a  variable  low  resistance  can  be  con- 
nected in  series  with  R.  This  resistance  usually  replaces 
the  oo  plug.  To  get  the  resistance  of  a  bridge  rheostat 
arm,  substitute  for  the  test  lines  a  piece  of  bare  copper 
wire  whose  dimensions  can  be  easily  determined.  The 
amount  of  this  wire  included  between  the  "AT"  binding 


174  TESTING    OF    DYNAMOS    AND    MOTORS. 

posts  can  be  varied  till  it  just  balances  the  resistance  of 
the  arm.  Its  own  resistance  is  then  determined  either 
from  a  wire  table  or  by  calculation  based  upon  its 
dimensions  and  specific  resistance:  a  constant  to  be 
considered  later.  In  ordinary  bridge  work,  the  rheostat 
arm  resistance  can  be  neglected  in  comparison  with 
that  of  X. 

Another  very  satisfactory  method  of  measuring  low 
resistances  is  by  means  of  the  differential  galvanometer. 
This  is  a  galvanometer  having  two  coils,  which,  when 
connected  either  in  series,  or  in  parallel  and  in  opposi- 
tion, should  have  equal  and  opposite  effects  upon  the 
needle  which,  of  course,  then  remains  at  rest.  The 
differential  action  depends  upon  the  magnetic  opposition 
of  the  coils  when  an  equal  current  is  sent  through  both 
but  in  opposite  directions.  If  the  coils  are  identical  as 
regards  number  of  turns,  and  in  position  as  regards  the 
needle,  complete  balance  is  secured  when  the  coils  are  in 
series,  because  the  current  through  both  is  necessarily  the 
same.  For  a  balance  with  the  coils  in  parallel  the  resist- 
ances must  be  the  same,  otherwise  the  same  current  will 
not  flow  through  both.  To  test  for  symmetry  of  position 
connect  the  coils  in  opposed  series,  and  apply  a  cell;  upon 
pressing  the  key  a  deflection  to  one  side  or  the  other 
indicates  one  coil  to  be  magnetically  stronger  than  the 
other.  A  controlling  magnet  should  be  used  to  bring 
the  needle  back  to  zero,  or  an  adjusting  coil,  sometimes 
provided,  brought  into  use.  Some  such  method  of 
compensation  is  necessary,  for  any  lack  in  the  coil  itself 
cannot  be  conveniently  remedied.  The  equality  of 
resistance  is  tested  by  connecting  the  coils  in  opposed 
parallel.  If  the  resistances  are  equal  the  currents  will 


MEASUREMENT    OF    RESISTANCE. 


'75 


be  the  same,  and  on  a  magnetically  balanced  instrument 
the  deflection  will  be  zero.  This  is  seldom  the  case,  how- 
ever, and  compensation  is  secured  by  placing  a  resistance 
box  in  series  with  the  coil  of  lower  resistance  and  adding 
resistance  till  the  needle  returns  to  zero.  Having  deter- 
mined the  value  of  this  added  resistance  a  special  coil 
can  be  made,  and  permanently  attached  to  the  base  of 
the  galvanometer.  If  this  coil  is  non-inductively  wound, 
that  is,  so  wound  that  there  are  as  many  turns  from  right 
to  left  as  from  left  to  right,  it  will  in  no  way  disturb 
the  magnetic  balance.  The  non-inductive  winding  is 
secured  by  doubling  the  wire  in  the  middle  and  winding 
it  on  double,  as  is  done  with  resistance  box  coils. 

There  are  two  schemes  of  connections  which  can  be 
used  in  measuring  resistance  with  the  differential  galvan- 
ometer.    Fig.    66    shows   the    method    best   adapted    to 
medium    or    low  resistances. 
The    coils    are     here    joined 
i  n  opposed  series;  t  h  e  u  n  k  now  n 
resistance,  A',  is  placed  as  a 
shunt    across    one   coil  while 
the  variable  resistance,  R,  is 
across     the     other.      If     the 
galvanometer  is    balanced  to 
begin   with,    it   will     be    bal- 
anced when  R  =  X\  for  under 

this  condition  the  shunting  power  of  each  is  the  same, 
and  equal  but  opposite  currents  still  pass  through  coils 
c  and  c '.  The  delicacy  of  this  method  depends  upon  the 
sensitiveness  of  the  galvanometer  and  upon  the  accuracy 
of  R.  The  test  consists  in  varying  R  till  the  needle 
returns  to  zero.  The  method  is  due  to  Heaviside.  It 


FIG.  66. 


176  TESTING    OF    DYNAMOS    AND    MOTORS. 

sometimes  happens  that,  just  as  on  a  bridge,  an  exact 
balance  cannot  be  obtained,  because  J?'s  units  are  too 
large.  In  this  case  we  calculate  the  balance  point  by 
interpolation.  Observe  the  lowest  possible  4-  deflection, 
say  it  is  10  divisions.  Next,  put  one  unit  into  R  or 
take  one  out,  as  the  case  may  be,  to  get  the  lowest 
possible  —  deflection;  say  it  is  25  divisions.  For  the  -j- 
deflection  supposed?  =  50  ohms  and  for  the  —  deflection 
R  —  51  ohms.  Then  the  total  deflection  25  -f-  10  =  35 
divisions  is  caused  by  a  difference  of  i  ohm  in  R;  1/35 
ohm  will  cause  a  deflection  of  i  division,  so,  in  order  to 
bring  the  needle  back  to  zero,  10/35  =  -2&  ohm,  which  is 
to  be  added  to  the  50  ohms,  making  50. 28ohms  the  point  of 
balance,  and  the  value  of  X.  r,  the  resistance  to  be  added 
to  the  smaller  value  of  R  in  any  case,  can  be  got  from 
the  proportion.  7?'s  unit  :  r  \  \  d  -|-  d'  :  d,  where  d  is 
the  -f-  deflection  and  d  -\-d'  the  total  deflection  caused 
by  a  difference  of  i  unit  in  R. 

For  measuring  high  resistances  the  coils  should  be  con- 
nected in  parallel,  X  being  in  series  with  one  and  R  with 
the  other.  Fig.  67  shows  the  plan  of  connections.  Under 
these  conditions,  too,  a  balance  obtains  when  R  =  X. 
Where  perfect  balance  cannot  be  obtained,  interpolation 
is  resorted  to.  It  is  well  to  introduce  the  box  r  into  the 
battery  circuit  to  cut  the  current  down  if  necessary. 
This  method  can  be  given  a  wide  range  of  measurement 
by  introducing  the  shunts  Sl  and  S^  which  should  be 
either  standard  shunts,  or  accurate  resistance  boxes  with 
a  total  resistance  about  equal  to  that  of  the  coil  which 
it  is  to  shunt.  As  the  two  galvanometer  coils  are 
alike  so  will  the  shunts  be.  Better  still,  have  St  and  S^ 
so  designed  with  respect  to  the  coils  as  to  give  the 


MEASUREMENT    OF    RESISTANCE. 


standard  ratios  of  1/9,  1/99  and  1/999.  If  this  method  is 
adopted  it  is  well  to  adjust  the  coils'  resistance  to  some 
even  figure,  and  to  order  the  shunt  boxes  accordingly. 
In  Fig.  67  it  can  be  seen  that  current  from  B  passes 
around  c  from  left 
to  right  but  around 
(•'  from  right  to 
left.  If  the  galvan- 
ometer coils  are 
electrically  and 
magnetically  bal- 
anced no  deflec- 
tion will  be  gotten 
when,  with  R  and 

A"  cut  out,  the  key  FIG.  67. 

is  pressed.    Placing 

X  in  the  circuit  with  c>  balance  is  destroyed  but  can  be 
restored  by  introducing  into  r"s  circuit  a  resistance 
equal  to  that  of  X.  A  perfect  balance  established,  R  = 
X.  To  eliminate  any  error  likely  to  arise  from  want  of 
a  proper  balance  between  the  coils  themselves,  it  is  only 
necessary  to  substitute  for  X  a  second  standard  box  and 
find  what  resistance,  R'  just  balances  R  as  already  found 
If  R'  =  R,  the  measurement  is  correct:  if  R'  does  not 
equal  R  it  still  equals  AT,  for  it  has  replaced  X  under  the 
same  conditions. 

The  smallest  resistance  to  be  had  in  R  is  perhaps 
i  ohm;  the  largest  10,000  ohms.  According  as  A"  is 
greater  than  10,000  or  less  than  i  ohm  shunts  Sl  and 
S^  respectively  are  used.  For  example,  suppose  it  is 
desired  to  measure  an  armature  whose  resistance  is 
.001  ohm.  It  cannot  be  measured  without  shunting  the 


178  TESTING    OF    DYNAMOS    AND    MOTORS. 

galvanometer,  for  there  is  in  R  no  resistance  small 
enough  to  balance  it  on  equal  terms.  X  being  much 
less  than  R  a  large  current  flows  through  coil  c  and 
causes  a  deflection;  before  the  needle  can  be  brought 
to  zero,  the  current  in  c  must  be  made  the  same  as  that 
in  c',  and  this  can  be  done  either  by  reducing  R  till  it 
equals  X,  or  by  shunting  coil  c  so  that  of  the  total 
current  which  flows  through  the  circuit  only  an  amount 
equal  to  that  in  c'  flows  through  c,  the  rest  going 
through  shunt  S9.  The  shunt  to  be  used  is  dictated  by 
the  degree  of  accuracy  desired.  .  Using  the  1/99  shunt 
the  removal  of  i  ohm  from  R  has  the  same  effect  as 
i/ioo  of  an  ohm  would  have  were  no  shunt  used;  and 
with  the  1/999  shunt,  the  same  as  1/1,000  ohm  would 
have  without  a  shunt.  For  suppose  that  with  no  shunt, 
the  removal  of  i  ohm  from  R  caused  a  decrease  of  i  am- 
pere in  the  current  flowing  through  circuit  r  M  N  B. 
The  whole  variation  takes  place  in  coil  c'.  If  only  1/1,000 
of  the  current  in  r  c  X  B  flowed  around  c,  a  variation  of 
i  ampere  in  r  X  B  would  cause  a  variation  of  approxi- 
mately but  1/1,000  of  i  ampere  in  coil  c.  This  condition 
is  more  properly  explained  but  less  easily  understood  by 
considering-  not  the  change  which  takes  place  in  c  when 
R  is  varied,  but  the  change  which  takes  place  in  c', 
because  if  we  suppose  a  constant  potential  to  be  main- 
tained between  points  M  and  Nt  variation  in  R  will  cause 
no  change  in  <r's  current.  However,  using  the  1/999 
shunt  S^  the  effect  of  <r's  coil  is  but  1/1,000  of  what  it 
would  be  were  no  shunt  used,  so  the  indicated  value  of 
X  at  time  of  balance  must  be  divided  by  1,000.  The 
object  attained  then  by  using  S9  is  to  reduce  the  effect  of 
each  ohm  in  R,  so  that  R  can  be  used  for  measuring  re- 


MEASUREMENT    OF    RESISTANCE.  179 

sistances  outside  its  own  range,  and  with  greater  accuracy 
those  within  its  range.  If  A"  is  many  times  greater  than 
R  the  shunt  must  be  used  in  c'  circuit;  then  instead  of 
dividing  7?'s  value  at  balance  by  the  shunt  power  used  we 
multiply  it.  Suppose  X  to  be  102,000  ohms.  Without  a 
shunt,  R  would  have  to  contain  102,000  ohms  to  balance 
this,  but  by  using  5,  =1/999  we  get  a  balance  with  R  — 
102  ohms.  In  general,  calling  6"  the  resistance  of  the 
shunt,  G  that  of  the  galvanometer,  R  the  standard,  and  A" 
the  unknown;  when  A"  >  R  (is  greater  than  R)  and  c9 
is  shunted,  X  —  R  X  the  shunting  power  of  S}.  When 
X  <  R  (X  is  less  than  R}  and  c  is  shunted,  A'  =  R  -^ 
shunting  power  of  52.  Here  R  means  the  resistance 
with  which  a  balance  is  gotten.  More  briefly  expressed, 
If 

A  >  R,  c'  shunted,  A  =   i  --     ~-c  X 

Cr 


If 

s 

X  <  R,  c  shunted,  X  =  „—.-$   X  R, 

&  +\ 

where 

_S_ 

G  +  S 

is  the  multiplying  power  of  the  shunt  and  is  gotten  by 
inverting  the  fraction  denoting  the  part  of  the  whole 
current  which  passes  through  the  galvanometer  coil.  In 
its  most  general  form  the  formula  giving  the  value  of  X 
in  terms  of  R,  G,  G'  S  S'  is 

'  + 
*  =  ** 


1+3, 

where  £and  G'  are  the  coil  resistances  and  6"  S'  that  of 


l8o  TESTING    OF    DYNAMOS    AND    MOTORS. 

their  respective  shunts.      If  G  and  G'  are  of  the  same  re- 
sistance then  G  —  G  and 


If  5  =    oo,  that  is  if  no  shunt  is  used  on  G, 
y-       R      -s' R 

/ ""  —  /~*    i      o/ " 
Cr          Cr  — j—  o 

^~S' 
Further  if 


G -\- S'       1,000'  1,000 ' 

If  no  shunts  are  used  both  S  and  S'  —  oo    and  X  =  R. 
To  investigate  it  a  little  more  mathematically:  if  G  is 
the  resistance  of  the  galvanometer  coil,  and  S  that  of  its 
shunt,  their  resistance  in  multiple  is 

G   S 

and  the  resistance  of  the  circuit  is 

G  S 

<^  +  - 
If  E\s  the  voltage  of  the  battery,  then 

I=E+(R  +  £± 


Also  we  know  that 


where  7g  is  the  current  in  the  galvanometer  coil.     Now 
it  is  this  coil's  current  which  interests  us: 


We  therefore  solve  for  fe  as  follows: 


MEASUREMENT    OF    RESISTANCE.  l8l 

(rr  a-  .<n  F  s  -|-  G 

,     '     — —  ••        r 


__         + 
therefore, 

(g  +  5) 


+  C)  +  £  5       S  +  ^  " 
SE 


£  (G  +  .S1)  +  G  S' 
Reducing  still  further  and  dividing  by  S,  we  get, 


'- 


The  meaning  of  this  last  equation  is  this:  the  value  of 
the  current  in  the  shunted  coil  is  the  same  as  it  would 
be  were  there  no  shunt  used,  and  if  instead,  J?'s  value 
were  increased  to 


S 
The  effect  of  the  shunt  then  is  to  multiply^  by 


s 

which  is  the  multiplying  value  of  the  shunt.  For  example, 
suppose  the  1/99  shunt  to  be  used  and  R=  5,249.  What 
is  AT?  Here  R  is  to  be  multiplied  by  100,  therefore  X  = 
5,249  x  ioo  =  524,900  ohms.  If  the  1/999  shunt  were 
used  and  R  to  balance  were  9,847  ohms,  X  would  be 
9,847  x  1,000  =  9,847,000  ohms.  When  X  is  smaller 
than  any  unit  in  R,  S,  is  used  and  the  formula  is 


l82  TESTING    OF    DYNAMOS    AND    MOTORS. 

x  =  *^s- 

Thus  suppose  the  1/99  shunt  is  used  and  R  =  45  ohms; 
then 

X  =  — —  =  .45  ohm. 
100 

Again,  with  the  1/999  shunt  and 

R  =  13,  X  =  —41—  =  -013  ohm. 


We  have  thus  increased  the  range  of  the  method  so  that  it 
is  useful  anywhere  from  1/1,000  of  the  smallest  unit  in  R 
to  1,000  times  the  total  value  of  R. 

Except  for  the  calculations  involved  in  reducing  results, 
the  use  of  an  ordinary  resistance  box  as  a  shunt  is  prefer- 
able to  the  standard  shunts,  as  it  admits  of  finer  adjust- 
ments. For  example,  suppose  G  =  500  ohms,  -5"  =  315 
and  R  =  9,950.  What  is  X? 


•  3r5        5°° 

x  =  —  —  x  R  =  6  \;r  -  x  9,950 

*J  3*5 

__  815  X  9,950   =  25)744  ohms. 
3i5 

If  a  1/99  shunt  had  been  used  the  balance  value  of  R 
would  have  been  257  and  the  value  of  X  found  to  be 
25,700  ohms,  or  44  ohms  too  small.  Had  the  1/9  shunt 
been  used,  R  would  have  balanced  at  2,574,  correspond- 
ing to  X  =  25,740  or  4  ohms  too  small.  So  we  see  that 
although  a  resistance  box  requires  more  work  than  a 
standard  shunt,  it  also  gives  greater  accuracy. 


CHAPTER   VII. 

MEASUREMENT    OF    INSULATION. 

WHEATSTONE'S  bridge  and  the  differential  galvanom- 
eter fulfill  the  requirements  for  a  wide  range  of  measure- 
ments, but  they  become,  less  and  less  accurate  as  this- 
range  is  increased.  For  very  low  resistances  the  "  Com- 
parison" method  is  much  the  best,  and  for  resistances 
reaching  up  into  megohms,  there  are  better  methods- 
which  we  will  now  investigate. 

Fig.  68  shows  the  connections  for  a  method  much  used: 
in  testing  insulation  resistance:  A  is  a  dynamo;  G,  a 
galvanometer,  with  shunt  5;  Ry  a  resistance  box»  of 
100,000  ohms  or  more  ;  r  is  a  standard  resistance,  and 
X,  the  resistance  to  be  measured.  R  is  necessary,  not 
only  to  set  up  the  galvanometer,  but  to  protect  it  in 
cases  where  X  is  found  to  be  defective.  For  a  test  of 
this  sort  to  be  thoroughly  reliable,  a  high  voltage,  (125 
to  500  volts)  must  be  used,  and  it  would  not  do  to  apply 
this  directly  to  the  galvanometer.  Switch,  K,  makes  it 
possible  to  throw  either  X  or  r  into  circuit  at  will. 
The  test  consists  in  first  noting  the  deflection  with  X  in 
circuit:  it  is  well  to  start  with  the  1/999  shunt  in;  and 
if  the  deflection  is  zero  or  small,  successively  put  in 
the  1/99  an<3  1/9  shunts,  and  finally  remove  the  shunt 
entirely.  If  the  deflection  is  now  zero  the  insulation  is 

183 


184 


TESTING    OF    DYNAMOS    AND    MOTORS. 


WWvVJ 
y 

FIG.  68. 


above  the  maximum  range  of  the  galvanometer,  for  the 
given  voltage,  and  may  be  considered  perfect.  When 

changing  shunt,  care 
must  be  taken  that  K  is 
open,  otherwise  the  gal- 
vanometer may  get  a 
current  that  will  twist 
the  fibre  several  turns, 
or  even  twist  the  needle 
off.  If  a  deflection  is 
obtained,  its  value  is 
noted;  this  we  will  call 

dj..  Next  throw  r  into  circuit,  and  if  it  is  possible, 
get  a  readable  deflection  with  the  same  shunt  as  used 
with  X.  In  this  case  the  following  proportion  holds: 
X  :  r  ;  ;  4  :  </x;  whence 

X  —  A  r. 

Suppose  r  =  100,000  ohms;  d^  =  50  divisions,  and  dY  = 
240  divisions.  Then 

240 

~5^ 

If  a  shunt  is  used  with  either  X  or  r  the  formula  must 
be  modified  accordingly.  Suppose  the  1/999  shunt  is 
used  with  r,  and  the  1/9  shunt  is  used  when  X  is  in  cir- 
cuit, then 


r 

X  =  —.  -  r  = 


X  100,000  =  4,800,000  ohms. 


The  range  of  measurement  then  extends  from 
r 


1,000 


to   1,000  r, 


MEASUREMENT    OF    INSULATION.  185 

or  from  100  ohms  to  100,000,000  ohms  where  r  =  100,000 
ohms.  Another  method  of  regulating  the  deflection  is 
to  keep  a  constant  shunt  on  the  galvanometer  and  vary 
the  voltage,  E,  of  dynamo  A.  In  this  case  E  is  varied 
till  the  same  deflection  is  obtained  from  A'  and  r,  when, 
A"  :  r  \  ;  E^.  :  Er\  and 


Suppose  £x   —   500;  Ev   -  •    50  and   r   —   100,000;    then 

soo 
X  —  -"     -  X   100,000  =  10,000,000  ohms. 

5° 

A  simple  and  inexpensive  method  of  varying  E  is  shown 
in  Fig.  69.  The  total  voltage  of  A  is  applied  to  the  high 
resistance  wire,  R  R  '.  One 
galvanometer  circuit  termi- 
nal is  secured  at  R  and 
the  other  is  fastened  to  the 
movable  contact,  c.  The 
voltage  in  the  galvanometer 
circuit  is  varied  by  sliding  c 
along  the  wire.  The  con- 
ditions to  be  filled  by  the 
wire  R  R'  as  regards  uni- 

formity of  cross-section,  etc.,  are  the  same  as  those 
discussed  under  the  slide  wire  bridge.  The  total  length 
of  R  R  should  be  divided  into  as  many  equal  parts  as 
there  are  volts  on  A\  then  any  desired  voltage  below 
this  can  be  gotten  by  taking  the  proper  proportion 
of  R  R"s  length.  Where  R  R'  is  very  long,  100  feet  or 
more,  it  must  be  mounted  in  some  manner  that  will 
make  every  inch  of  it  easily  accessible.  To  do  this  it 
can  either  be  wound  back  and  forth  on  wooden  pins 


l86  TESTING    OF    DYNAMOS    AND    MOTORS. 

driven  into  a  board,  or  it  can  be  wound  in  a  spiral  on 
a  wooden  drum  with  brush  contacts  at  either  end.  In 
both  arrangements  R  R'  should  be  graduated  in  volts 
and  the  values  marked  at  intervals  on  the  board  or  drum 
just  beneath  the  wire.  If  there  are  500  volts  on  A,  the 
galvanometer  circuit  will  be  subjected  to  the  full  500  if 
the  free  end  c  rests  on  R  ';  to  250  volts,  if  c  rests  mid- 
way between  R  R\  and  so  on,  the  fraction  of  the  total 
voltage  being  the  same  as  the  fraction  of  the  total  length 
of  R  R'  included.  For  this  reason  it  is  more  convenient 
to  mark  R  R'  in  fractions  of  its  own  length,  then  the 
marking  is  good  whatever  may  be  the  voltage  of  dynamo 
A.  It  may  not  always  be  convenient  to  conduct  the 
test  by  varying  E,  in  which  case  recourse  must  be  had  to 
the  method  of  variable  shunts  described  above. 

To  be  strictly  accurate  the  resistance  of  generator,  gal- 
vanometer and  R  must  be  considered.  When  X  is  in  cir- 
cuit (see  Fig.  68)  its  resistance  maybe  considered  so  high 
as  to  require  no  shunt  to  get  a  readable  deflection;  in  this 
case  the  deflection  is  inversely  proportional  to  the  cir- 
cuit resistance,  and  if  A's  voltage  is  £,  we  may  write 


, 

A  +  G  +  R  +  X  ' 

When  r  replaces  X,  a  shunt  generally  has  to  be  used, 
because  r  is  so  much  less  than  X  that  the  current  sent 
through  it  by  E  throws  the  ray  off  the  scale.  The  effect 
of  this  shunt,  as  has  been  shown,  is  to  virtually  increase 
the  circuit  resistance 

S+  G 
~S~ 

times,  so  that  we  may  write, 


MEASUREMENT    OF    INSULATION.  187 

E 


'.= 


From  these  values  of  d^  and  dl  we  get  by  division, 
E  E 


{,  '    A+G+R+X 


O 


: 


•   A  +  G  +  R  +  X  £ 

_  ^^  (A  +  r  +  R)  +  G    .    • 

A  +  G  +  R  +  X 
Clearing  of  fractions  and  dividing  by  d^  we  have 


Whence  solving  for  X,  we  get 


where  A  =  generator  resistance,  and  1?,  r,  £and  S,  that 
of  the  constant  box,  standard  resistance,  galvanometer 
and  shunt  respectively.  X  is  the  unknown  resistance. 
When  r  and  X  are  large  A  and  G  may  be  neglected  and 
the  formula  becomes, 


If  X  and  r  are  alone  taken  into  account,  then 


l88  TESTING    OF    DYNAMOS    AND    MOTORS. 

„ d^        S  -(-  G 


which  reduces  it  to  the  case  already  discussed.     If  we 
use  the  1/999  shunt, 

S+  G 


and 


X-    *• 
X-      t 


=  1,000 


1,000  r 


These  methods  are  used  in  the  commercial  testing  of 
glass  and   porcelain   insulators  and   marine  cables.     To 


FIG.  70. 

test  insulators  they  are  arranged  along  a  shelf  containing 
holes  to  receive  them.  Each  one  is  then  filled  with  water 
to  within  an  inch  of  the  top,  and  the  tank  is  filled  to  the 
same  level.  They  are  then  allowed  to  stand  several 
days  to  give  the  water  every  chance  to  creep  into  any 
crevices  which  might  exist.  Fig.  70  shows  the  connec- 
tions for  the  test  which  then  follows.  The  tank,  T,  con- 
tains the  insulators,  #,  a,  a.  The  test  lines  are  fastened 
one  to  the  iron  tank  and  the  other  to  a  long  ebonite  rod 
so  as  to  be  movable.  By  means  of  the  two-way  switch,  K, 
the  galvanometer  may  be  thrown  on  the  lines  L  L'  or  on 
the  standard  high  resistance  box  R.  The  other  appur- 


MEASUREMENT    OF    INSULATION.  189 

tenances  are  the  same  as  those  already  noted.  The  test 
consists  in  throwing  the  galvanometer  alternately  upon 
Z  Z',  the  test  lines,  one  of  which  touches  the  water 
through  the  tank  case,  the  other  of  which  touches  the 
water  in  the  insulator,  and  upon  the  box  R.  The  deflec- 
tions obtained  are  then  compared.  Let  </f  be  the  deflec- 
tion due  to  R,  and  d^  that  due  to  Ar,  the  insulation 
resistance.  If  the  1/999  shunt  be  used  with  R.  we 
have 

X  ~- ''  4~  \  (B  +  r  +  R)  S  \G  +  G  \  -  (G  +  B  -f  r) 
«•    (  &  ) 

or  with  sufficient  accuracy 

A'  —  1,000  —j-  R. 

Throughout  the  test,  the  lines  Z  Z'  should  be  exchanged 
to  detect  any  insulation  defect  in  the  testing  system. 
Before  and  after  the  test,  line  Z  should  be  held  in  air  and 
K  closed.  Any  deflection  shows  faulty  line  insulation, 
which  should  be  remedied.  It  is,  however,  impossible  to 
maintain  perfect  insulation  in  the  pressure  of  such  high 
voltage;  perfection  on  a  dry  day  often  proving  faulty  oi\ 
a  wet  one.  All  wiring  and  devices  should  be  supported 
on  rubber  cleats  and  legs,  and  these  should  be  carefully 
dusted  daily.  Besides  that  caused  by  surface  leakage  due 
to  moisture,  there  will  be,  on  closing  the  key,  a  momen- 
tary deflection  due  to  the  line's  capacity;  the  line  takes  a 
static  charge,  the  amount  of  which  depends  upon  the 
potential  of  the  battery  and  the  line's  capacity. 

The  effects  of  capacity  depend  on  the  well-known  fact 
that  if  we  connect  two  points  which  are  at  different 
potentials,  a  flow  of  electricity  takes  place  between  them 


IpO  TESTING    OF    DYNAMOS    AND    MOTORS. 

until  the  points  are  at  the  same  potential.  If  some 
agent  is  employed  to  keep  the  points'  potentials  dif- 
ferent, at  the  same  rate  that  the  current  equalizes  them, 
we  have  a  continuous  flow  of  current,  as  in  an  ordinary 
active  lighting  or  power  circuit  where  the  external  flow 
of  current  represents  the  -(-  and  —  brushes'  effort  to 
equalize  their  potentials,  while  the  armature  is  the  agent 
which  maintains  the  potential  difference.  Now  if  a  lot  of 
dead  wiring  be  tapped  on  to  one  brush  of  a  live  dynamo 
this  wiring  will  acquire  the  potential  of  that  brush,  and 
in  doing  so  causes  a  momentary  flow  of  current,  whose 
strength  and  duration  will  depend  upon  how  much  elec- 
tricity it  takes  to  raise  the  wires  to  the  same  potential  as 
the  rest  of  the  system. 

In  the  above  case  all  the  wires  on  one  side  of  the 
key  are  attached  to  the  battery  or  dynamo,  and  are  at  a 
certain  potential:  wires  on  the  other  side  of  the  key  are 
dead  or  at  zero  potential,  except  in  so  far  as  they  may 
preserve  a  charge  from  their  last  communication  with  the 
live  side.  As  soon  as  the  key  is  closed  there  is  an 
equalization  of  potential,  which  sends  a  momentary  cur- 
rent through  the  galvanometer  and  causes  a  deflection. 
This  deflection,  however,  should  quickly  subside,  whereas 
a  deflection  due  to  leakage  will  persist  and  be  of  constant 
value,  and  must  be  subtracted  from  the  deflections 
obtained  from  the  box  R,  and  from  the  insulation  X,  in 
order  to  get  the  true  deflection  due  to  these  resistances 
alone.  In  the  above  test  the  insulators  are  first  rapidly 
tested  by  tapping  the  stalk  with  the  terminal  L.  Those 
measuring  below  a  certain  value  are  removed,  and  the 
balance  are  then  connected  in  groups  of  10  or  100  by  con- 
necting all  the  stalks  together.  The  insulation  resist- 


MEASUREMENT    OF    INSULATION.  191 

ance  of  these  groups  is  then  taken  accurately.  The  pres- 
ence of  a  poor  insulator  in  a  group  will  bring  down  the 
average  of  the  group;  on  the  other  hand  a  lot  of  good  in- 
sulators may  boost  a  moderately  poor  one  through  :  hence 
the  preliminary  test.  The  surface  leakage  of  the  insulators 
taken  together  further  lowers  the  apparent  value  of  the 
insulation.  This,  however,  is  the  condition  under  which 
the  insulators  are  used,  and  hence  is  the  test  which  they 
should  be  called  upon  to  stand.  The  average  insulation 
per  insulator  is  required  to  be  above  250,000  megohms, 
and  anything  below  200,000  megohms  is  rejected.  If 
100  insulators  are  in  a  group  and  they  are  assumed  to  be 
of  equal  insulating  power,  their  total  resistance  con- 
nected in  parallel  will  be  i/ioo  of  that  of  any  single 
insulator.  If  200,000  megohms  is  the  minimum  for  a 
single  insulator,  200,000  -r-  100  =  2,000  megohms  is  the 
minimum  for  the  group. 

In  measuring  the  insulation  of  marine  cables  and 
cables  for  underground  work,  the  same  method  is 
resorted  to.  The  cable  is  coiled  in  a  tank  6  or  7  feet 
deep,  and  its  ends  are  brought  out  and  supported  in  mid- 
air by  means  of  ebonite  rings.  Water  is  then  run  into  the 
tank  and  the  cable  left  there  for  several  days.  Each  end 
is  then  stripped  of  its  insulation  and  lead,  or  other 
sheathing,  for  a  few  inches  back,  so  as  to  increase  the 
length  of  the  surface  between  the  lead  and  the  copper. 
To  further  decrease  surface  leakage  the  ends  are  pencil- 
shaped,  and  coated  with  hot  paraffine  before  being  tested. 
The  longer  a  cable,  the  less  will  be  its  total  insulating 
power.  Contracts  therefore  require  not  that  the  whole 
cable  have  a  certain  insulation  resistance,  but  that 
it  have  a  certain  insulation  resistance  per  foot.  Since 


192  TESTING    OF    DYNAMOS    AND    MOTORS. 

the  total  insulating  power  diminishes  with  the  length 
of  the  cable  (since  every  foot  adds  a  path  for  leakage), 
the  average  insulation  per  foot  is  gotten  by  multiplying 
the  total  insulation  by  the  number  of  feet, 


or,  Insulation  of  the  whole  cable  =  Lnsulation  Pe. 

Length  in  feet 

Hence,  as  above,  insulation  per  foot  =  total  insulation  x 
number  of  feet.  Cables  are  in  general  of  three  types: 
single  conductor  cables,  double  and  triple  conductor  ca- 
bles, and  telephone  cables.  In  the  first  the  insulation  is 
measured  from  the  conductor  to  the  lead;  in  the  second, 
between  conductors,  which  are  concentric,  and  from  the 
outer  one  to  ground;  in  the  third,  between  each  of  the  52 
to  104  pairs  of  conductors.  That  is,  each  wire  is  tested 
with  every  other.  They  are  all  then  twisted  together 
and  tested  to  ground.  In  telephone  cables  the  insulation 
need  not  be  very  high  from  wire  to  wire,  since  the 
voltage  at  which  they  will  work  is  not  high.  The  test  is 
more  for  the  purpose  of  insuring  that  the  cable  be  free 
from  crosses.  Even  in  case  of  a  cross  the  defective  wire 
is  simply  ignored;  it  is  on  this  account  that  several  extra 
wires  are  put  in  each  cable,  so  that  a  100  wire  cable  has 
104  wires.  The  insulation  to  ground  is  tested  at  a  lower 
voltage  than  in  high  tension  cables. 

High  tension  cables  are  subjected  to  a  further  test  :  the 
cable  is  put  into  a  tank,  and  the  secondary  of  a  step  up 
transformer  connected  to  the  cable  and  the  tank.  The 
voltage  is  then  raised  to  a  value  several  times  above  that 
to  which  the  cable  will  be  subjected  in  practice.  If  it 
stands  this  test  the  voltage  is  lowered  to  a  point  some- 
what exceeding  that  at  which  the  cable  is  to  work,  and  it 


MEASUREMENT    OF    INSULATION.  193 

is  allowed  to  stand  for  an  hour.  At  the  end  of  this  time 
the  insulation  is  measured  by  the  above  method. 

The  method  also  admits  of  the  locating  of  a  fault.  To 
do  this  it  is  only  necessary  to  leave  one  terminal  of  the 
testing  lines  on  one  end  of  the  cable  the  other  remaining 
on  the  tank.  The  cable  is  then  slowly  drawn  from  the 
tank.  As  soon  as  the  faulty  spot  leaves  the  water  the 
insulation  improves,  as  shown  on  the  galvanometer:  after 
verifying  the  results  by  repeating  the  test,  the  cable  is 
cut  off  at  the  faulty  point  provided  the  portion  left  is 
long  enough  to  be  of  any  use. 

Another  method,  well  adapted  to  the  measurement  of 
high  resistance  insulation,  and  equally  applicable  to 
underground  or  overland  lines,  depends  upon  the  elec- 
trometer, explained  above.  The  method  of  using  it  in  this 
test  is  as  follows  and  is  illustrated  in  Fig.  71.  The  elec- 
trometer, £,  has  one  side  connected  to  tank  T,  and  the 
other  to  cable,  C.  The  cable  is  first  connected  to  a 
source  of  E.  M.  F.,  say  a  500  volt  line,  and  is  given  a  static 
charge  the  value  of  which  depends  upon  the  cable's 
capacity.  The  electrometer  is  then  switched  onto  the 
cable.  The  plates  now  acquiring  the  potential  of  C 
and  T  respectively,  attract  each  other  and  cause  a 
deflection  which  keeps  its  initial  value  so  long  as  the 
cable's  charge  remains  the  same.  The  charge  will 
remain  constant  if  C's  insulation  is  perfect:  if  not,  leak- 
age takes  place,  the  deflection  becomes  smaller  and 
smaller,  and  becomes  zero  in  the  course  of  half  an  hour 
or  more,  according  to  the  value  of  the  insulation.  In 
line  work  the  method  is  not  as  good  as  those  already 
described,  for  an  electrometer  has  not  the  range  of  use  of 
a  galvanometer,  and  is  hence  a  relatively  more  expensive 


194 


TESTING    OF    DYNAMOS    AND    MOTORS. 


instrument.     We   do   not,  therefore,   give   the   equations 
involved  in  the  method. 

In  measuring  comparatively  low  insulations,  such  as 
are  found  in  armatures,  commutators,  and  fields,  a  Weston 
voltmeter  can  be  used  to  advantage.  It  is  accurate,  por- 
table, needs  no  setting  up,  and  can  be  used  with  any  voltage 
within  the  range  of  its  scale.  Fig.  72  gives  the  connec- 


FIG.  71. 

tions.  B  is  the  source  of  E.  M.  F.;  V,  a  500  volt  voltmeter; 
A,  an  armature  whose  insulation  from  copper  to  iron  is  to 
be  measured,  and  K,  a  switch  by  means  of  which  A  can 
be  cut  out  and  the  voltmeter  constant  taken.  Here  the 
meter  resistance  takes  the  place  of  that  of  the  galvanom- 
eter and  boxes.  The  test  consists  in  first  closing  K  and 
observing  F's  deflection.  This  deflection  will  be  the  full 
voltage  of  B  and  is  called  the  "constant."  If  B  is  a  500 
volt  dynamo,  the  constant  will  be  the  full  scale  reading, 
or  500,  and  is  due  to  the  current  which  500  volts  will  send 
through  the  resistance  of  the  meter.  Suppose  this  resist- 
ance to  be  80,000  ohms,  =  Rv.  If  upon  introducing  the 
insulation  resistance,  X,  into  circuit,  and  opening  K,  the 
deflection  drops  to  250,  just  one-half,  we  know  that 
the  current  through  Fhas  been  halved,  and  that  since 


MEASUREMENT    OF    INSULATION.  195 

the  voltage   has  been   unchanged,  the  circuit  resistance 
has  been  doubled  and  is  now  160,000  ohms.     Taking  from 
this  F's    resistance  of   80,000 
ohms,    we     have    left     80,000 
ohms  as  that  of  X.      If  d^  the 
deflection    with    X   in  circuit, 
falls   to  one-quarter  of  </,,   we 
know  X  -j-  Rv  has    become  4 
X     80,000    ohms     =     320,000  FJG.  72. 

ohms,  and    that  A'  is  320,000 

-80,000  ::  240,000  ohms.  In  any  case  inserting  X 
makes  dl  as  many  times  d^  as  the  resistance  of  X  -{-  -#T 
is  times  that  of  J?v  alone.  In  other  words  to  get  the 
value  of  AT  -f  7?v,  multiply  ^  by 


This  gives 


if  we  take  from  this  the  meter  resistance  ^T,  we  have  left 
that  of  the  insulation,  or 


which  is  the  same  as 
X 


-•(ft-) 


and  is  the  working  formula.  Using  the  same  letters^ 
we  will  derive  this  formula  a  little  differently.  Since 
dl  and  d^  are  proportional  to  the  currents  which  cause 


196  TESTING    OF    DYNAMOS    AND    MOTORS. 

them,  we  can  in  this  case   substitute  them  for   these  cur- 
rents and  write, 

/r* 

<  =  §7 

and 


dividing  the  first  by  the  second,  we  get, 

A        ^y  +  ^        A    ,    _*"  ,X 

<*,''        £,        :  R, +  ^v  =        "  ^ ' 


or 


~~7~  —  I  ~\     TT  > 

' 


whence 

TT     /?          I  '  I 

Thus,    let  ^v  =  80,000   ohms,    as  before;  dl  =  500,  and 
</a  —  3.      Then 

'•^(i-')-*-.^-)- 

/Soo  —  3\ 
80,000  I  -  ~  I  ==  80,000  x  165  =  13,200,000  ohms, 

or  13.2  megohms. 

If  Rv  •=  8,000;  di  =  200,  and  d^  =  20; 

/  200          \ 
Jf  =  8,000  I i  J   =  8,000  X  9  —  72,000  ohms, 

a  very  low  insulation. 

*  More  correctly  d\  =  -77-  >i',  but  the  constant  £  may  be  omitted  with- 


out error. 


MEASUREMENT    OF    INSULATION.  197 

Now  suppose^  =  80,000;  dl  —  500,  and  </a  =  500  also: 
then 

A'  --  80,000  |   :          -  i    J  =  80,000  (i  --  i) 

=  80,000  X  o  =  o, 

which  shows  that  A*  is  no  insulation  at  all:  /.  e.,  that  the 
copper  actually  touches  the  iron.  It  is  now  customary  to 
put  a  voltmeter  at  the  disposal  of  the  workman,  so  that  he 
can  test  his  work  in  its  different  stages  of  construction,  and 
thereby  detect  a  fault  as  soon  as  it  occurs,  and  thus  to 
avoid  having  it  returned  after  its  completion.  This  saves 
time,  labor,  and  material.  It  is  then  necessary  to  tell  the 
field  or  armature  winder,  or  commutator  builder  what  de- 
flection will  pass  his  work.  Suppose  Rv  —  80,000,  E  =  500, 
and  it  is  deemed  safe  to  pass  any  work  whose  insulation 
hot  is  1,000,000  ohms  or  i  megohm.  What  is  the  maxi- 
mum deflection  (</a)  allowable?  The  value  of  X  must  not 
be  below  1,000,000  :  this  is  to  say  X  -|-  R^  must  not  be 
less  than  1,080,000  ohms.  We  saw  above  that  X  -f-  R*  is 
always  as  many  times  Rv  as  dl  is  times  d9.  How  many 
times  Rv  is  X  -\-  Rvl  X  —  1,000,000;  Rv  —  80,000;  A'  -f- 
R^  —  1,080,000 

^4-  ^v  _    1,080,000  _ 


R  80,000 

d^  then,  can  be  13^  times  d^  d^  must  be  1/13^  of  dl  = 
I/I3I/4  of  5°°  =  i/V-  of  500  =  fa  of  500  =  37.  A 
deflection  of  37  with  X  in  circuit  means,  then,  an  insula- 
tion resistance  of  i  megohm.  37  is  the  maximum  allow- 
able deflection  when  E  =  500.  If  E  =  1,000,  d9  may  be 


190  TESTING    OF    DYNAMOS    AND    MOTORS. 

74;  if  E  =  125,  d^  must  be  no  more  than  9^  divisions. 
To  get  this  result  more  direct  take  the  formula 


?H 


X  = 


and  substitute  for  the  letters  their  numerical  values,  and 
we  get 


1000000 


/5°°          \ 
=  80,000  i  —  i  1 


dividing  both  sides  by  10,000,  to  simplify,  we  get 


100    — 


or  100  d^  =  8  (500  —  </3)  :  100  </3  =  4,000  —  8  d^  whence 
1  08  d^  —  4,000  and 


as  before.  Any  piece  of  insulation  letting  a  deflection  of 
over  37  take  place  through  it  would  be  rejected.  In  prac- 
tice, it  is  customary  to  dispense  with  switch  A"  and  to  get 
d^  by  holding  the  test  lines  together.  One  line  is  then 
held  to  the  shaft  and  the  other  on  to  the  commutator  if  the 
armature  is  connected  up.  If  the  commutator  alone  is  to 
be  tested  one  line  goes  to  shell  and  the  other  is  passed 
from  bar  to  bar:  any  bar  giving  a  deflection  of  over  37 
is  marked,  afterward  examined,  and  if  necessary  replaced. 
The  next  test  is  that  of  insulation  between  bars.  This 
need  not  be  high,  since  under  working  conditions  the 
difference  of  potential  between  adjacent  bars  is  only  a 
fraction  of  the  machine's  E.  M.  F.  The  connections  are 


MEASUREMENT    OF   INSULATION.  Ip9 

given  in  Fig.  73.  Test  lines  T  T'  are  first  crossed  and 
*/,  obtained.  They  are  then  touched  to  the  same  bar;  if 
d^  is  the  same  it  shows  that  there  is  no  oil  or  shellac  on 
the  commutator  to  influence  the  readings.  The  lines  are 
now  gotten  such  a  distance 
apart  as  to  span  one  mica 
body,  without  touching  the 
same  bar  at  the  same  time. 
To  start  with,  one  mica  body 
is  proven  to  be  all  right  and 
is  then  short  circuited  with  a  Fui.  73. 

lead-pencil   mark.      The    lines 

are  now  held  stationary,  the  commutator  slowly  turned 
and  the  deflection  noted  each  time  a  body  is  spanned  by 
the  lines.  When  the  pencil  mark  is  reached  after  making 
a  complete  revolution  the  needle  will  indicate  a  short 
circuit.  The  lead  mark  makes  it  unnecessary  to  watch 
the  commutator,  so  that  the  attention  can  be  centered  on 
the  needle.  When  a  "  low  body  "  is  spanned,  the  needle 
shows  it  by  a  deflection  whose  magnitude  depends  upon 
the  nature  of  the  trouble:  A  burr  on  the  "  ear  "of  a 
bar,  or  a  filing  touching  both  bars  will  give  the  full  deflec- 
tion. A  burr  may  be  found  by  inspection  but  a  filing 
may  be  so  imbedded  as  to  give  great  trouble.  If  the 
fault  is  not  too  serious  it  can  often  be  burnt  out  with 
current  from  a  lamp  circuit.  Holding  one  terminal  on 
one  bar,  the  other  terminal  is  tapped  on  the  next  bar; 
if  the  test  lines  are  held  on,  matters  may  be  made  worse, 
for  when  the  cross  gives  way  an  arc  may  be  set  up  and 
the  insulation  charred,  or  a  bead  of  copper  deposited 
between  the  bars.  If  the  contact  is  too  "good  "  to  burn 
out  the  commutator  must  be  dismounted,  and  the  faulty 


20O  TESTING    OF    DYNAMOS    AND    MOTORS. 

insulation  replaced.  This  means  that  the  nut  and  washer 
must  be  removed,  several  bars  taken  out,  new  insulation 
put  in,  the  parts  reassembled,  the  bars  gauged  again  to 
see  that  they  lay  straight,  the  commutator  turned  down 
and  baked  again,  and  then  retested.  This  will  impress 
upon  the  mind  of  the  most  skeptical  the  advisability  of 
having  the  builder  test  his  work  in  course  of  construction 
with  something  less  delusive  than  a  magneto. 

Where  a  commutator  shows  a  uniform  high  deflection 
all  around,  it  indicates  moisture,  and  must  be  baked  more. 
On  repaired  commutators  this  symptom  is  due  sometimes 
to  the  soldering  acid  forming  a  bridge  between  bars  or  to 
the  insulating  bodies  having  become  carbonized.  In  the 
first  case  the  mica  bodies  must  be  well  scraped  between 
the  "  ears  "  where  the  acid  is  apt  to  have  crept.  In  the 
second  case,  the  chances  are  that  carbonization  has  taken 
place  where  the  commutator  is  exposed  to  the  joint 
action  of  the  paraffine  and  sparking  of  the  brushes,  and  a 
light  cut  off  the  commutator  will  generally,  though  not 
always,  remove  the  difficulty.  Where  a  commutator  tests 
perfect,  with  the  exception  of  20  or  30  bars,  it  indicates 
moisture;  shellac  may  have  accumulated  in  a  space  be- 
tween the  bars  and  shell,  and  there  being  so  much  of  it, 
it  would  require  longer  to  dry  it  out.  The  most  difficult 
trouble  to  locate  is  where  it  is  due  to  poor  quality  of  the 
insulating  material  itself.  This  fault  can  only  be  located 
by  the  negative  method  of  proving  every  other  source  of 
trouble  to  be  absent.  The  manufacturers  themselves  are 
fooled  sometimes,  and  warrant  goods  that  prove  to  be 
worthless.  Inferior  grades  of  tape,  shellac,  varnish,  and 
rnica  sometimes  give  no  end  of  trouble,  and  are  mislead- 
ing, because  they  pass  the  test  when  new,  but  do  not  stand 


MEASUREMENT    OF    INSULATION.  *2Ol 

service.  Another  source  of  trouble,  sometimes  unavoid- 
able, is  the  practice  of  having  the  winding  room  and 
commutator  department  exposed  to  the  iron-filings  and 
flying  dust  of  machine  shops  and  emery  wheels.  The 
writers  know  of  several  instances  where  much  trouble 
was  traced  to  such  surroundings. 

This  method  of  measuring  insulation  by  means  of  a 
Weston  voltmeter  is  being  adopted  everywhere,  and  it 
would  be  a  great  convenience  if  the  makers  would  furnish 
with  each  instrument  a  chart  or  curve,  giving  the  insula- 
tion corresponding  to  each  deflection  when  a  specified 
voltage  is  used.  This  is  a  matter  of  interest  to  all,  and 
not  only  to  those  unfamiliar  with  formulae. 

It  must  be  remembered  that  the  insulation  value  ob- 
tained by  any  of  the  above  methods  is  by  no  means  a 
constant  definite  quantity,  but  varies  according  to  the 
voltage  used  in  the  test.  Insulation,  perfect  under  a 
test  of  500  volts,  would  not  be  so  when  subjected  to  r,ooo. 
This  is  because  the  higher  voltage  not  only  has  a  greater 
ability  to  break  down  insulation,  but  by  increasing  the 
leakage,  the  instrument  is  affected  by  a  current  that  passes 
over  and  not  through  the  insulation.  It  is  very  impor- 
tant that  the  voltage  used  in  the  test  should  at  least 
equal  that  at  which  the  machine  is  to  work.  The  writers' 
experience  has  been  that  commutators  can  be  built  to 
stand  the  500  volt  test  from  bar  to  shell  without  disturb- 
ing the  needle  at  all,  and  from  bar  to  bar  with  only 
a  slight  wiggle  of  the  needle.  If  an  insulation  from  bar 
to  shell  of  i  megohm  can  be  considered  enough,  that 
from  bar  to  bar  should  be  at  least  one-half  as  much.  On 
armatures,  especially  of  the  toothed  type,  and  on  fields 
it  is  much  more  difficult  to  perfect  the  insulation  on 


202  TESTING    OF    DYNAMOS    AND    MOTORS. 

account  of  the  increased  surface  exposed  to  the  iron. 
On  a  smooth-core  armature,  only  the  bottom  layer  of 
wire  is  exposed  to  the  core,  but  on  toothed  types  every 
wire  is  next  to  either  the  bottom  or  side  of  a  slot.  As  an 
example  of  the  effect  of  increased  surface,  we  cite  the 
instance  of  an  unconnected  armature,  each  of  whose  coils 
tested  perfect  alone;  when  they  were  all  connected 
together  by  means  of  a  piece  of  No.  18  bare  copper  wire 
wound  around  the  ends,  the  voltmeter  showed  a  deflec- 
tion of  20;  upon  cutting  the  No.  18  wire  so  as  to  divide  the 
armature  into  two  insulated  halves,  each  half  gave  a  de- 
flection of  10 ;  each  quarter  of  5,  and  so  on.  At  present 
it  is  customary  to  perfect  the  insulation  of  each  part 
before  it  goes  into  the  machine.  The  completed  machine, 
after  being  subjected  to  a  severe  running  test,  is  again 
tested  for  insulation;  this  should  be  taken  about  ten 
minutes  after  the  machine  is  shut  down,  for  while  run- 
ning, fanning  of  the  air  keeps  the  armature  surface  cooler 
than  the  interior;  and  insulation  should  be  tested  only 
when  everything  is  at  its  uniform  maximum  temperature. 
This  fact  accounts  for  what  may  seem  strange,  that  many 
armatures  burn  out  five  or  ten  minutes  after  being  shut 
down,  and  it  also  explains  how  an  armature  hastily  tested 
and  pronounced  sound,  sometimes  shows  up  a  cross  or 
ground  when  it  is  attempted  to  put  it  into  service  out  on 
the  road.  It  has  always  been  the  writers'  custom,  where 
proper  testing  instruments  were  wanting,  to  run  a 
machine  its  regular  test,  allow  it  to  cool  off,  and  run  it 
again  for  a  short  while. 

As  it  is  often  convenient  to  use  a  galvanometer  for 
conducting  the  above  tests  we  will  give  some  suggestions 
in  regard  to  galvanometer  practice.  What  can  be  said 


MEASUREMENT    OF    INSULATION. 


203 


of  either  voltmeter  or  galvanometer,  can  with  few  excep- 
tions be  said  of  both.  All  lines  in  any  way  connected 
with  an  insulation  test  must  be  run  on  porcelain  or  hard 
rubber  insulators  to  minimize  leakage,  and  to  prevent 
grounds.  The  presence  of  such  faults  in  testing  systems 
are  fruitful  sources  of  trouble,  resulting  in  deflections  of 
unwonted  value,  sometimes  in  blowing  fuses,  and  some- 
times in  twisting  the  needle  from  its  suspension. 

Fig.  74  illustrates  the  connection  of  the  above  voltmeter 
test.     K  is  a  reversing  switch  to  galvanometer  G.     Jt  is 
a  constant      resist- 
ance; A  is  the  ar- 
mature whose  insu- 
lation to  shaft  is  to 
be  measured.    Sup- 
pose the    shaft   to 
be  resting  on  iron 
horses  and  hence  to 

be  in  communication  with  the  ground  at  e.  In  the  event 
of  a  second  ground  occurring  on  the  line  its  effect  would 
depend  upon  its  location.  If  a  fault  is  at  elt  current  from  B 
will  pass  direct  from  71,  down  through  ^,  up  through  elt  and 
back  to  B.  B  having  both  terminals  directly  to  ground  at 
e  and  elt  the  galvanometer  and  box,  1?,  are  both  cut  out. 
If  £'s  voltage  is  high,  as  is  usually  the  case  in  insulation 
testing,  some  part  of  the  line  must  give  way,  and  for  this 
purpose  a  fuse  should  be  introduced  at  B.  If  B  is  a  few 
cells  of  battery  the  only  effect  is  a  low  or  zero  deflection 
on  the  galvanometer  for  both  positions  of  K.  If  a  fault 
occur  at  e^  It's  resistance  and  that  of  A's  insulation  are 
cut  out,  leaving  the  galvanometer  in  circuit  with  only 
its  fuse  to  protect  it  from  injury.  In  such  a  case,  the 


204 


TESTING    OF    DYNAMOS    AND    MOTORS. 


proper  step  is  to  interchange  the  lines  T and  T' ;  i.  e., 
the  one  on  the  shaft  goes  to  the  commutator  and  vice 
versa  :  the  effect  of  this  is  to  place  both  grounds  on  the 
same  side  of  By  and  if  the  accidental  ground  is  at  <?v,  R 
is  cut  out,  leaving  the  armature  alone  in  circuit  with  the 
galvanometer.  If  X,  the  insulation  resistance,  is  high 
compared  to  R,  the  deflection  will  not  be  appreciably 
affected  as  long  as  X  is  in  circuit;  but  if  7"and  T'  should 
happen  to  touch,  B  would  short  circuit  through  G  and 
perhaps  injure  it. 

Next  suppose  the  ground  to  be  at  ez\  the  deflection  in 
this  case  would  be   the  same  as   if  the  test  lines   were 

held  together,  for  the 
armature  insulation, 
X,  is  cut  out  ;  by 
interchanging  T  and 
T',  ground  e  and  ez 
are  placed  on  the 
same  pole  of  B,  and 
the  deflection  returns. 
to  its  proper  value. 

It  is  thus  seen  that  the  question  of  insulation  in 
a  testing  system  is  important.  The  arrangement  of 
Fig.  75  is  an  improvement  over  that  of  Fig.  74.  In 
this  arrangement,  the  insulation  of  G  and  R  can  be 
easily  made  perfect,  and  a  ground  on  the  lines  either  at 
el9  e^  or  «?3  cannot  cut  out  R  and  harm  G.  For  the  given 
position  of  the  lines  a  ground  at  et  will,  as  before,  short 
circuit  B  and  cause  a  deflection  lower  than  the  ^con- 
stant." If  B  is  a  high  resistance  cell  and  the  constant 
gotten  by  holding  T  and  T'  together  is  small,  the 
difference  in  deflection  caused  by  such  a  ground  is  apt 


e, 


FIG.   75. 


MEASUREMENT    OF    INSULATION. 


205. 


to  pass  unnoticed  unless  the  ground  makes  a  dead  short 
circuit,  in  which  case  the  galvanometer  circuit  is  sub- 
jected only  to  the  drop  which  takes  place  through  the 
fault.  Short  circuiting  a  source  of  high  voltage  will 
hardly  pass  unnoticed,  as  more  or  less  violent  demonstra- 
tions attend  such  a  fact.  The  proper  proceeding  in  the 
above  case  is,  as  before,  to  interchange  the  test  lines. 
A  ground  at  en  will  have  no  effect  with  the  connections 
given,  unless  the  lines  get  interchanged,  when  the  case  of 
a  ground  at  <?3  is  re- 
peated. A  ground  at 
cy  cuts  out  A  and 
gives  the  "  constant  " 
deflection.  Inter 
changing  T  and  T' 
removes  the  trouble. 
Fig. 76  gives  a  plan  for 

over    that    of   Fii 


«  . 


further   improvement 


. 


Ik-re    all 


connections  save  the  test  lines  are  included  in  a  compact 
system  which  can  be  well  insulated  and  permanently 
mounted.  K  now  reverses  only  T'and  T'  and  does  not 
affect  £'s  current.  If  it  is  desired  to  reverse  6"s  cur- 
rent, as  is  often  the  case  where  we  wish  to  get  a  deflec- 
tion on  both  sides  of  zero  in  order  to  use  the  average,  the 
reversal  can  be  done  by  a  second  switch  to  which  the  gal- 
vanometer lines  run,  as  in  any  of  the  foregoing  diagrams. 
The  effect  of  a  ground  on  T  or  T'  is  to  cut  out  A,  which 
is  indicated  by  the  deflection  being  the  constant.  Such  a 
fault  is  quickly  detected  by  interchanging  the  test  lines; 
hence  the  advantage  of  having  K  as  in  Fig.  76,  where 
reversal  of  T  and  T'  is  controlled  by  the  galvanometer 
operator.  Otherwise  there  must  be  some  signal  for  tell- 


2O6  TESTING    OF    DYNAMOS    AND    MOTORS. 

ing  the  "  lineman  "  to  interchange  T  and  T'.  If  both 
test  lines  are  grounded  or  otherwise  crossed,  a  deflection 
will  obtain  even  when  T  and  T'  are  held  apart.  Where 
a  man  is  experienced  in  galvanometer  work  he  is  not  apt  to 
reject  sound  insulation  which  tests  apparently  low  through 
the  influence  of  a  line  fault,  especially  where  high  voltage 
is  used,  for  he  is  always  more  or  less  disagreeably  reminded 
of  the  presence  of  the  fault  when  any  part  of  his  person 
comes  in  contact  with  a  key,  switch,  or  other  device 
which  is  on  the  ungrounded  side  of  the  circuit.  On 
a  test  circuit  where  several  cells  of  battery  are  used 
as  the  source  of  E.  M.  F.,  the  operator  gets  no  shock 
even  if  he  touches  the  two  leads  from  the  battery; 
with  the  result  that  if  one  side  of  the  circuit  is  already 
grounded,  as  at  e^  in  Fig.  75,  and  he  should  stand 
on  the  wet  floor  and  touch  the  key,  K,  on  the  upper 
side  he  would  produce  a  ground  virtually  at  et,  cut 
out  A  and  make  it  appear  low.  Personal  contact 
with  any  part  of  a  low  voltage  testing  circuit  will' 
introduce  error  in  any  of  the  foregoing  tests.  The 
operator  must  then  keep  clear  of  exposed  parts  of  the 
circuit,  and  the  "  lineman  "  must  not  only  keep  his  hands 
off  the  test  line  terminals  but  must  be  very  careful  that 
the  points  do  not  rest  on  dirt,  lacquer,  or  grease,  and 
that  they  are  not  burnt  to  an  oxide  on  the  end.  Where 
high  voltages  are  used  these  errors  due  to  personal  con- 
tact are  not  so  apt  to  occur.  Grounds  at  <?4  and  eb  in 
Fig.  76  should  be  of  very  rare  occurrence,  and  are  readily 
located.  The  above  cited  complications  which  arise  in 
galvanometer  practice  as  a  result  of  defective  insulation 
are  representative,  and  most  others  are  greater  or  lesser 
modifications  of  them.  A  little  experience  on  a  faulty 


MEASUREMENT    OF    INSULATION.  207 

testing  circuit  will  soon  impress  the  tester  with  the 
importance  of  good  insulation. 

The  tendency  of  the  day  seems  to  be  toward  the  use  of 
high  voltages,  because  to  transmit  a  given  amount  of 
energy  to  any  distance,  the  line  loss  is  much  less  where 
the  energy  is  transmitted  as  high  voltage  and  low  current, 
than  where  low  voltage  and  high  current  are  employed; 
for  instance  suppose  we  wish  to  transmit  one  horse 
power  (i  HP)  a  distance  of  one  mile  over  a  line  composed 
of  No.  10  B.  &  S.  copper  wire.  What  would  the  loss  be 
in  sending  i  ampere  current  at  746  volts,  and  what  would 
it  be  with  746  amperes  at  i  volt,  the  voltage  in  both 
cases  being  that  at  the  user's  end  of  the  line  ?  i  Mile  = 
5,280  feet.  The  resistance  of  1,000  feet  No.  10  B.  & 
S.  copper  wire  is  i  ohm:  the  resistance  of  5,280  feet  is 
5.28  times  i  ohm  =  5.28  ohms.  Since  there  is  a  return 
wire  the  line  is  really  2  miles  long  and  its  resistance  2  x 
5.28  =  10.56  ohms.  Now  from  Ohm's  law,  E  =  I  R, 
we  have  E  —  i  x  10.56  =  10.56  volts  as  that  necessary 
to- send  the  current  of  i  ampere  through  the  line;  the 
loss  =  El  =  10.56  x  i  —  10.56  watts.  In  the  second 
case  /  =  746  and  R  =  10.56,  and  E,  the  drop  consumed 
in  sending  the  current,  is  I R  —  746  x  10.56  —  8,877 
volts,  making  the  enormous  loss  of  10.56  x  8,877  = 
90,000  watts  approximately.  This  is  selected  as  an 
exaggerated  case  to  bring  out  the  difference  boldly.  A 
No.  10  B.  &  S.  wire  will  not  carry  746  amperes. 

Large  electrical  companies  have  a  very  perfect  system 
of  testing  insulation  intended  to  work  with  high  voltages, 
and  in  this  system  the  tests  above  cited  are  usually 
preliminaries.  Before  subjecting  the  insulation  to  an 
alternating  voltage  test  of  from  1,000  to  5,000  volts  (some- 


208  TESTING    OF    DYNAMOS    AND    MOTORS. 

times  much  higher  than  this),  it  is  tested  by  galvanometer 
or  voltmeter  at  500  volts,  or  less,  to  insure  the  absence  of 
moisture  and  other  immediate  causes  of  short  circuit. 
Fig.  77  gives  the  connections  for  a  high  voltage  test. 
In  this  particular  test  the  voltage  from  the  generator  was 
1,050  and  was  brought  to  transformer,  T,  which  reduced 
it  to  50  volts.  In  cases  where  a  50  volt  alternating  cur- 
rent lighting  circuit  is  available,  T  can  be  dispensed  with. 
J3  is  a  combination  switch  and  fuse  box  by  means  of 
which  all  communication  with  the  live  source  can  be  cut 
off.  T's  50  volt  secondary  passes  to  switch  K.  C  is 
a  circuit  breaker,  which  flies  out  when  poor  insulation 
permits  the  current  to  exceed  a  certain  value.  A  is  an 
amperemeter,  and  from  here  passes  one  side  of  the  sec- 
ondary circuit  of  the  stationary  transformer,  7",  to  one 
side  of  the  primary  of  the  variable  transformer,  T' 
The  other  side  of  T's  secondary  runs  direct  from  switch 
K  to  the  remaining  primary  terminal  of  T'.  Now  the 
primary  of  T'  slides  in  and  out  of  the  secondary  by 
means  of  the  windlass  at  P,  and  it  is  in  this  manner 
that  the  secondary  voltage  is  varied  from  100  to  5,000 
volts.  The  theory  being  that  when  the  primary  is  inside 
the  secondary  all  of  the  latter's  coils  are  cut  by  the 
primary's  lines  of  force,  whereas  when  outside,  only  a 
few  of  the  lines  reach  it  and  its  voltage  is  correspondingly 
low.  To  the  terminals  of  T "s  secondary  the  test  lines 
are  attached  and  are  applied  to  the  insulation  to  be  meas- 
ured. In  this  case,  however,  the  test  lines  are  not  held 
on  by  hand,  but  are  fastened  on  and  the  circuit  closed  and 
opened  at  K.  Before  K  is  closed,  T"s  primary  must 
be  at  zero  as  indicated  by  the  arrow.  Closing  K,  P 
is  turned  so  as  to  run  up  the  primary  into  the  secondary 


2IO  TESTING    OF    DYNAMOS    AND    MOTORS. 

until  the  insulation  under  test  either  gives  way  or  stands 
the  full  test.  Windlass  P  had  best  be  acted  upon  by  an  op- 
posing weight,  which  will  automatically  return  the  coil  to 
the  zero  position  should  the  operator  carelessly  leave  it  in 
the  position  of  high  voltage.  V  is  a  voltmeter  attached 
across  the  secondary  of  7",  and  registers  50  volts  at  time  of 
test.  An  electrometer  is  used  (not  shown  in  Fig.  77)  for 
registering  the  voltage  on  T"s  secondary,  and  it  is  this 
that  the  tester  watches  when  testing.  As  soon  as  switches 
B  and  K  are  closed,  ammeter  A  registers  the  current 
which  the  50  volts  send  against  the  self-induction  of  T'  's 
primary.  If  the  insulation  under  test  is  perfect,  no  cur- 
rent flows  through  T"s  secondary,  there  is  no  reaction 
on  its  primary,  and  A's  current  does  not  increase;  but  if 
the  insulation  at  X  breaks  down  current  flows  in  T"s 
secondary,  its  lines  of  force  spread  out  and  cut  the 
primary  turns,  reduce  the  induction,  and  let  the  50  volts 
send  a  larger  current  through  the  primary  circuit.  If 
the  breakdown  is  very  bad,  C  flies  out  and  opens  the 
circuit.  In  such  a  test  all  apparatus  should  be  well  pro- 
tected from  passers-by. 

It  is  not  always  convenient  to  bring  the  electrical 
parts,  armatures,  fields,  transformers  and  the  like,  to  a 
testing  room  to  be  tested,  so  to  obviate  this  inconvenience 
some  of  the  leading  manufacturers  have  adopted  the  fol- 
lowing very  flexible  plan.  In  Fig.  78,  P  is  the  primary 
coil  of  a  transformer  which,  being  connected  to  a  plug,  can 
be  inserted  in  any  lamp  socket  on  the  shop  alternating 
current  lighting  mains.  The  secondary  coil  has  several 
leads  brought  out,  and  by  means  of  these  different  sec- 
ondary potentials  can  be  gotten.  By  using  the  two  end 
leads  the  voltage  is  raised  to  3,000:  leads  i  and  3  give 


MEASUREMENT    OF    INSULATION. 


211 


2,000;  3  and  2,  1,000;  and  soon,  in  as  many  steps  between 
the  limits  as  there  are  extra  leads.  In  this  case,  no 
voltage  indicator  is  needed  on  the  secondary  coil  because 
the  transformer  is  usually  designed  for  a  particular  pri- 
mary voltage,  that  of  the  shop,  and  this  assumed,  the 
various  secondary  voltages  can  be  marked  under  their 
respective  leads  on  the  box.  Small  testing  sets  can  be 


:>  1 

1000 

<    ^   30( 

0 

1 

FIG.  78. 

carried  by  hand,  but  the  larger  ones  of  greater  range, 
containing  coils  immersed  in  oil,  are  heavy,  and  must  be 
pushed  from  shop  to  shop  on  a  truck. 

The  question  of  liquid  resistance  arises  in  connection 
with  batteries  and  water  rheostats.  In  very  accurate 
work  it  is  necessary  to  know  the  battery  resistance.  If 
the  resistance  of  a  column  of  any  liquid  is  known  and 
taken  as  a  unit,  it  is  easy  to  estimate  the  dimensions  of  a 
box  to  fulfill  any  conditions  of  rheostat  service. 

The  Wheatstone  bridge  affords  a  ready  means  of  apply- 
ing a  battery  to  the  measurement  of  its  own  resistance. 
Fig.  79  shows  the  connections  for  such  a  test.  Here  the 
battery  occupies  the  bridge  arm  in  which  we  usually  find 
the  unknown  X :  but  in  this  test  the1  battery  is  not  only 
X  but  is  the  source  of  E.  M.  F.  as  well.  The  method 
differs  a  little  from  the  ordinary  bridge  method,  in  that 
balance  is  effected  with  a  constant  deflection  rather  than 


212 


TESTING    OF    DYNAMOS    AND    MOTORS. 


a  zero  deflection.     The  arms  P  and  Q  are  the  proportion 

.arms  and  their  values  once  chosen  are  not  changed.  The 

galvanometer  circuit  being 
always  closed  there  is  a  cur- 
rent and  deflection,  R  is  al- 
tered till  upon  closing  K  the 
deflection  is  not  changed  in 
value.  Under  this  condi- 
tion closing  K  does  not  cause 
a  redistribution  of  poten- 
tial over  the  system,  and 
hence  does  not  disturb  the 

galvanometer,  and  we  have  the  proportion/5  :  Q  ;  \  R  :  X, 

whence 

X  =  Q-  x  R. 

One  objection  to  the  method  is  that  any  polarization  of 
the  battery  will  raise  its  resistance;  on  the  other  hand  this 
has  the  advantage  of  being  the  condition  of  actual  ser- 
vice. The  error  due  to  polarization  can  be  eliminated 
either  by  using  a  sensitive  galvanometer  so  that  only  a 
small  current  passes  or  by  employing  other  methods. 
When  the  battery  resistance  is  very  small,  such  as  that 
of  a  storage  cell,  the  following  method  is  recommended. 
A  Weston  amperemeter  and  voltmeter  serve  the  pur- 
pose in  the  connections  of  Fig.  80,  where  V  is  a  volt- 
meter of  such  high  resistance  as  to  have  practically  no 
shunting  power,  whence  its  current  can  be  neglected  and 
the  reading  of  A  called  /?'s  current.  R  is  a  resistance. 
Call  E,  /?'s  open  circuit  E.  M.  F.,  and  E'  its  terminal 
E.  M.  F.  as  indicated  on  V,  when  the  current  in  A  is  /. 


MEASUREMENT    OF    INSULATION. 


:•  ; 


Now  when  current  flows  the  E.  M.  F.  of  B  is  divided 
into  two  parts  —  that  which  drops  through  the  external 
resistance  and  which  it  is  unnecessary  to  calculate 
because  Vindicates  it;  and  that  which  drops  through  r, 
the  battery  resistance,  and  equals  by  Ohm's  law  /  r. 
Then  E  =  E'  -j-  /  /•  whence  E  —  E'  =  I  r  and 


The  test  consists  in  taking  E,  then  closing  K  to  get  E' 
and  I  ;   this  method  is  adapted  to  industrial  work.* 

Liquid  resistance  can  be  found  by  confining  the  liquid 
in  a  glass  tube  having  at  its  ends  metal  electrodes,  one  of 
which  can  be  moved  along  the  tube  to  vary  the  length  of 
the  liquid  column.  In  Fig.  8r  E  is  the  movable  elec- 


1  ' 

1  «  ir- 

L^_l 

$71 

L$E 

FIG.  So. 


FIG.  81. 


trode,  G  a  low  reading  ammeter,  and  R  a  resistance 
adjusted  to  give  a  convenient  current  value.  £'s  position 
being  noted,  A'  is  closed  and  7  quickly  read  on  G.  The 
reading  must  be  made  quickly  to  avoid  the  error  of 
polarization.  E  is  than  moved  along  a  few  inches  and  a 
new  reading  taken.  I  is  now  less  than  before;  now,  by 
means  of  R  restore  7  to  its  former  value.  If  .ffand  E' 
have  been  pushed  nearer  together,  in  order  to  reduce  7 
to  its  first  value  we  must  introduce  in  R  a  resistance 
*  For  other  methods  of  measuring  battery  resistance  see  some  standard 
treatise,  such  as  that  of  Carhart  and  Patterson. 


214 


TESTING    OF    DYNAMOS    AND    MOTORS. 


equal  to  that  of  the  column  of  liquid  included  between 
the  first  and  second  position  of  E.  Knowing  the  dimen- 
sions of  this  column  we  can  estimate  the  resistance  of 
any  column  of  the  same  liquid.  It  is  well  to  start  with 
E  at  about  the  middle  of  the  tube  and  to  draw  it  out, 


FIG.  82. 

thus   increasing   the  resistance,  using  smaller   currents 
and  minimizing  the  effects  of  polarization. 

Water  rheostats,  or  water  boxes  as  they  are  commonly 
called,  are  made  upon  this  principle.  In  place  of  the 
tube  is  a  long  trough  as  in  Fig.  82,  or  a  box  or  barrel,  as 
in  Fig.  83,  which  should  rest  upon  insulators,  but  seldom 
does.  The  electrodes  are  iron  plates,  one  of  which  is  free 
to  slide  in  guides  as  in  Fig.  82,  or  free  to  move  up  and 
down  as  in  Fig.  83.  The  resistance  of  the  rheostat  is 
varied  either  by  varying  the  dimensions  of  the  liquid 
column  (moving  the  plates  or  putting  in  or  taking  out 
some  water)  or  by  improving  its  conductivity  or  impair- 
ing it  (putting  in  salt  or  soda  to  increase  conductivity; 
substituting  for  some  of  the  briny  solution  pure  water  to 
decrease  it).  The  water  box  finds  a  place  in  the  testing 
of  various  electrical  machines,  especially  in  providing 
converters  and  high  tension  alternating  current  machines 
with  non-inductive  loads — equivalent  to  a  lamp  load 
,  without  the  intervention  of  transformers.  The  water 


MEASUREMENT    OF    INSULATION. 


215 


box    finds  a  more  industrial    application    in    connection 

with  the  speed  regulation  of  turbines  and  is  growing  in 

favor    as    a    starting  box  for 

stationary    motors,    or     as   a 

permanent    resistance  in   cir- 

cuit with  a  low  voltage  motor 

running    from    high    voltage 

mains. 

The  foregoing  methods  of 
testing  liquid  resistances  are 
open  to  the  objection  that 
polarization  must  more  or  less 
modify  results.  The  follow- 
ing method  is  recommended 
as  free  from  this  objection. 
In  Fig.  84  A  is  an  ordinary 
slide  bridge  with  a  standard 
non-inductive  resistance,  j?, 

and  the  liquid  contained  in  tube,  L.  B  is  a  source  of 
alternating  current,  such  as  is  furnished  by  an  ordinary 
lighting  circuit,  and  Z"a  telephone  receiver  used  in  place  of 
a  galvanometer.  As  the  contact  S  is  moved  along  the  wire 
the  buzzing  in  the  telephone  becomes  louder  or  fainter 
as  the  case  may  be.  When  S  reaches  a  point  at  the  same 
potential  as  Aft  the  sound  ceases,  or  at  least  becomes  a 
minimum.  Under  this  condition  we  have: 


P: 


Other  forms  of  Wheatstone  bridge  are  not  so  well  suited 
to  this  test,  on  account  of  the  coils'  self-induction. 
which,  however  small,  cannot  be  said  to  be  zero. 


TESTING    OF    DYNAMOS    AND    MOTORS. 


The  method  is  adapted  to  the  determination  of  a  sub- 
stance's specific  resistance,  which  we  have  learned  is  the 

resistance  of  a  piece 
of  any  substance  us 
compared  with  the 
resistance  of  a  piece 
of  similar  dimensions 
of  another  substance 
taken  as  a  standard. 
The  standard  is  pure 
copper.  If  a  piece  of 
FIG.  84.  pure  copper  measures 

so  much  and  a  similar 

piece  of  some  other  substance  measures  twice  as  much, 
the  specific  resistance  of  the  latter  substance  is  said 
to  be  2. 

In  speaking  of  a  conductor's  resistance,  we  tacitly  in- 
volve not  only  the  conductor's  dimensions  and  specific  re- 
sistance, but  we  must  know  the  temperature  at  which  the 
measurement  has  been  made:  hence  in  compiling  tables  of 
resistances  it  is  customary  to  give  resistance  values  at 
some  accepted  standard  temperature.  This  standard  is 
usually  taken  as  15.5°  C.  or  60°  F.,  the  average  yearly 
temperature  near  London,  England.  Carbon  excepted, 
all  solid  conductors  rise  in  resistance  as  the  temperature 
rises,  but  with  carbon  and  liquids  the  reverse  is  true.  To 
find  the  amount  of  this  effect,  it  is  necessary  to  measure 
the  resistance  of  the  conductor  at  several  temperatures 
and  to  divide  the  change  of  resistance  by  the  change 
of  temperature.  The  result  is  ohms  per  degree,  and 
is  called  the  temperature  coefficient.  Very  careful 
experiments  have  been  made,  particularly  by  Dr. 


MEASUREMENT    OF    INSULATION.  2iy 

Mathiessen,  upon  copper,  showing  that  the  resistance  of 
a  pure  copper  conductor  changes  i  £  for  each  2.58°  C. 
change  in  temperature.  Thus  suppose  the  air  to  be  at 
i5.5k>C  and  that  at  this  temperature  a  machine's  shunt 
winding  measures  10  ohms.  What  will  it  measure  at 
77.5°  C. ?  The  change  of  temperature  is  77.5°  —  15.5° 
=  62°.  i  <?0  of  10  ohms  is  .1  ohm.  The  resistance 
increases  as  many  times  .  i  ohm  as  2.58  is  contained  in  62, 
and 

62 
--.-  =  24  times; 

and  24  x  .1  ohm  =  2.4  ohms  increase  in  resistance,  10 
ohms  -j-  2.4  ohnm  =  12.4  ohms,  the  resistance  at  77.5°  C. 
We  have  here  assumed  the  air  to  beat  standard  tempera- 
ture. Take  a  more  likely  case.  Let  R  be  13  ohms  at 
21. 5°  C.  What  will  it  be  at  15. 5°  C.?  Here 

^'-'5:5: 

2-5** 

i  %  of  13  =  .13  and  2.3  x  .13  =  .299  ohm  change  in 
resistance.  Now  since  the  final  temperature  is  less  than 
the  initial,  the  value  of  R  is  less,  so  we  must  subtract 
the  correction  from  the  initial  value,  and  we  have,  13  — 
.299  =12.7  ohms  at  standard  temperature.  In  these 
examples  the  change  in  temperature  is  given  and  the 
change  in  R  estimated.  It  is  more  often  that  we 
must  figure  the  final  temperature  from  the  rise  in  R, 
obtained  by  measuring  before  and  after  heating.  If  a 
field's  resistance  is  27.2  ohms  at  air  temperature  of  30°  C. , 
and  the  resistance  is  31.4  ohms  at  the  end  of  a  two-hour 
test;  What  is  the  final  temperature?  Since  R  rises  i  $ 


2l8  TESTING    OF    DYNAMOS    AND    MOTORS. 

for  every  2.58°  rise  in  T,  T  will  increase  as  many  times 
2.58°,  as  the  rise  in  R  is  times  i  %  of  the  initial  value  of  R. 
Initial  R  =  27.2  ohms.  Final  R  —  31.4  ohms.  31.4  - 
27.2  =  4.2  ohms  rise  in  R.  i  %  of  27.2  =  .272  and  4.2  -=- 
.272  =  15.4  times;  15.4  X  2.58°  =  39.7°  rise  in  T.  30°  + 
39.7°  =  69. 7°  final  temperature.  These  examples  show  that 
for  temperatures  below  the  standard  the  correction  is 
negative,  /".  <?.,  must  be  subtracted  from  the  resistance 
value  at  the  standard  temperature,  while  for  temperatures 
above  the  standard  the  correction  is  positive  and  is  to  be 
added.  These  facts  are  expressed  in  the  following  for- 
mula: R'  =  R  -J-  R(t—  15. 5°  C.)  k,  where  R  is  the  resis- 
tance after  test,  R  the  initial  resistance,  /  the  temperature 
at  which  the  resistance  is  desired,  and  k  the  percentage 
change  in  R  for  i°  C.  In  this  case,  considering  copper,  k 
=  .33  of  i  %. 

To  build  the  above  formula  up  from  first  principles  we 
proceed  as  follows:  Since  R  represents  the  final  value 
and,/?  the  initial  value  of  the  resistance,  R(t  —  15.5°  C.)  k 
must  represent  the  increase  in  resistance,  and  the  formula 
must  read,  the  final  resistance  =  the  initial  resistance 
-\-  the  increase  in  resistance.  Further,  let  us  analyze 
the  expression  which  denotes  the  increase,  namely 
R(t  —  15. 5°  C.)  k.  Here  /  —  15. 5°  gives  us  the  number 
of  degrees  rise  in  temperature.  Also  /£is  the  percentage 
of  R  which  a  rise  of  i°  in  temperature  causes  R  to  rise.  R 
X  k  is  then  the  actual  increase  in  ohms  which  i°  causes 
and  this  X  by  the  degrees  (/  —  15.5)  rise,  gives  (/  — 
15.5°  C.)  R  k;  whence,  R  =  R  -f  R  (t  —  15. 5°  C.)  k,  or 
R  —  R  [!_!_(/_  I5.5o  c.)k}.  If /is  greater  than  15.5,  R' 
is  greater  than  R.  If  /  is  less  than  15.5  the  expression 
R  (t  —  15.5°  C.)  is  negative  and  must  be  subtracted 


MEASUREMENT    OF    INSULATION.  219 

from  R  to  give  A'.     We  now  wish  to  know  the  value  of 
k.     For  copper  it  is  practically  = 

.01 

-  =  .0018  , 
2.58 

so  the  above  formula  becomes  R'  =  R  [i  -{-  (*  —  I5-S^  C.) 
.0038]. 

The  temperature  coefficient  of  alloys  is  generally  less 
than  that  of  its  component  metals.  This  decrease 
in  the  coefficient  suggests  the  possibility  of  finding  an 
alloy  with  a  zero  coefficient;  this  is  nearly  attained  in 
manganin— an  alloy  12  $  manganese,  84  %  copper,  4  % 
nickel.  The  following  table  shows  its  value: 

MANGANIN.* 

RANGE  OF  TEMP.  MEAN  TEMP.  COIL. 

10°  to  20° -[-.000025 

2O        "     30        -J-.OOOOI4 

30  "  35  4-  .000004 

35  "  40  4~  -000003 

40  "  45  ...   -j-  .000001 

45  "  50  --  .000001 

50  "  55  --.000002 

55  "  60  --  .000004 

60  "  65  —.000005 

It  is  here  seen  that  the  coefficient  is  positive  up  to 
45°  and  then  becomes  negative;  the  average  coefficient 
for  the  whole  range  is  -j-  .0000 1  and  for  a  range  of  from 
30°  to  65°  the  mean  value  is  zero. 

The  question  of  temperature  coefficient  is  important 
in  dealing  with  resistances  and  batteries,  for  where 

*  Dr.  Lindeck,  Proc.  International  Elect.  Congress,  1893,  p.  165. 


220  TESTING    OF    DYNAMOS    AND    MOTORS. 

accuracy  is  desired  it  is  now  customary  to  calculate  final 
temperatures  from  the  increase  in  7?,  instead  of  using  a 
thermometer,  for  the  latter,  being  in  contact  with  only 
the  surface  layers  of  the  winding,  registers  less  than  the 
true  value.  It  is  possible,  however,  to  perform  a  series 
of  experiments  which  shall  show  how  the  true  tempera- 
ture compares  with  that  indicated  by  the  thermometer. 
This  consists  in  carefully  measuring  a  resistance  at 
atmospheric  temperature  and  at  the  same  time  noting 
this  temperature  on  the  thermometer.  Next,  send  a 
small  current  through  the  coil  and  allow  it  to  get  as  hot 
as  that  current  will  make  it;  now  put  the  thermometer 
on  and  get  a  temperature  while  the  current  still  flows;  at 
the  same  time  measuring  R  so  that  from  the  rise  in  7?  the 
temperature  can  also  be  figured.  Next,  increase  the 
current  5  %  or  10  %  and  allow  the  coil  to  reach  its  maximum 
temperature  for  that  current.  Read  the  thermometer 
and  place  opposite  its  indication  the  true  temperature  as 
figured  from  the  rise  in  R.  Repeat  these  observations  a 
dozen  or  more  times  and  there  will  result  two  rows  of 
figures — the  thermometer  temperatures  and  the  corre- 
sponding true  temperatures,  which  we  will  call  respectively 
a  and  b.  To  be  most  useful  these  values  should  be 
plotted  in  a  curve,  where  a  is  laid  off  horizontally 
and  b  vertically.  By  establishing  several  points  by  means 
of  the  above  experiment  we  find  the  law  of  variation,  and 
are  justified  in  assuming  that  the  law  which  holds  for 
those  points  holds  for  any  points  in  between;  hence  the 
points  may  be  connected  by  a  curve.  To  find  what  value 
of  b  corresponds  to  an  undetermined  value  of  a,  draw  from 
the  a  point  in  question  a  perpendicular  line  to  cut  the 
curve  at  c\  from  c  draw  a  horizontal  to  the  b  line:  the 


MEASUREMENT    OF    INSULATION.  221 

intersection  with  this  line  is  the  value  of  £  sought.  Such 
a  curve  must  be  plotted  for  each  style  of  coil  which  the 
tester  has  ordinarily  to  do  with,  because  the  relation 
between  thermometer  readings  and  the  true  temperature 
is  not  the  same  on,  say,  a  field  coil,  of  an  open  motor, 
as  it  is  on  a  closed  one.  In  the  above  experiment  there 
must  be  in  circuit  with  the  coil  under  test,  a  current  in- 
dicator with  which  to  keep  the  current  constant  in  the 
successive  steps.  The  current  may  be  regulated  by 
means  of  an  auxiliary  resistance  in  series  with  the  coil,  or 
in  multiple  with  it,  provided  the  ammeter  is  cut  in  with 
the  coil  alone,  or  better  still  by  varying  the  voltage  on 
the  dynamo  which  supplies  the  current.  To  tell  when  the 
maximum  temperature  corresponding  to  any  given  cur- 
rent is  reached,  we  depend  upon  the  fact  that  when  the 
temperature  ceases  to  rise,  so  does  R,  for  R's  increase  is 
due  to  increase  in  temperature.  Now  if  a  voltmeter  be 
put  permanently  across  R,  its  deflection  will  always  be 
proportional  to  R,  and  when  R  rises,  so  also  will  the  de- 
flection. When  j?  ceases  to  rise  the  deflection  becomes 
constant,  and  we  know  that  the  temperature  is  at  its 
highest  value.  It  is  well  to  wait  five  minutes  after  the 
deflection  seems  to  be  constant  to  be  certain  of  it:  also 
to  be  sure  that  the  current  is  at  its  proper  value. 

We  know  a  conductor's  resistance  to  depend  upon  its 
length  and  cross-section,  and  we  have  just  learned  that 
it  also  depends  upon  temperature  and  specific  resistance. 
Specific  resistance  is  dependent  upon  quality,  and  is  of 
the  same  nature  as  specific  gravity,  in  that  both  express  a 
substance's  property  as  compared  with  the  same  property 
of  some  other  substance  taken  as  a  standard.  The  stand- 
ard of  specific  gravity  is  a  cubic  centimetre,  or  cubic  inch, 


222  TESTING    OF    DYNAMOS    AND    MOTORS. 

of  water  at  its  greatest  density.  If  a  cubic  inch  of  another 
substance  weighs  twice  as  much  as  a  cubic  inch  of  water 
the  specific  gravity  of  the  substance  is  2.  The  standard 
of  specific  resistance  is  a  cube  of  pure  copper  at  standard 
temperature.  If  the  resistance  of  a  similar  cube  of  an- 
other substance  measures  twice  as  much  as  that  of  the 
copper  one,  that  substance's  specific  resistance  is  2.  It 
is  impracticable  to  measure  accurately  the  resistance 
of  a  cube  and  moreover  very  inconvenient  to  make  one. 
If,  however,  we  know  the  resistance  and  dimensions  of 
any  regularly  shaped  conductor,  the  resistance  of  an  inch 
cube  can  be  deduced.  Suppose  the  resistance  of  1,000 
feet  of  No.  10  B.  &  S.  copper  wire  is  i  ohm.  Its  cross- 
section  is  .0082  square  inch.  It  will  take  as  many  such 
wires  to  make  up  a  cross-section  of  i  square  inch  as  .0082 
is  contained  in  unity,  or 

— - —  =  121.95. 
.0082 

The  resistance  of  1,000  feet  of  such  a  cable  is  then 

of  i  ohm  or  .0082  ohm: 

121.95 

the  resistance  of  i  foot  is  1/1,000  of  this  or  .0000082 
ohm,  and  of  i  inch  1/12  of  .0000082  or  .00000068  ohm:  N 
/.  <?.,  the  resistance  of  i  cubic  inch  of  the  same  copper  as 
the  wire  is,  is. 00000068  ohm.  If  the  copper  is  pure 
copper,  then  the  actual  resistance  of  the  standard  is  this. 
The  appended  table  gives  the  specific  resistance  of  the 
various  metals.  The  formula  for  finding  the  resistance 
of  any  conductor  is: 

R  =  TV^^T—    X  Spec.  Res.  X  .00000068, 
Cross-Sec. 

and  the  result  is  expressed  in  ohms. 


MEASUREMENT    OF    INSULATION. 
TABLE  OF  SPECIFIC  RESISTANCE.* 


223 


SUBSTANCE. 

RESISTANCE  OF 
I  CUBIC  INCH. 

RELATIVE 
RESISTANCE. 

METALS  AT  O°  C. 
Copper  (annealed) 

IN   MICROHMS. 

I  OO 

Copper  (hard) 

A,a 

I  OIQ 

Silver  (annealed)                              . 

•  U4J 

Q-  2A 

Silver  (hard)   .  .                 ... 

oy* 

6J7 

I  086 

Gold  (annealed)  

8  10 

i  -;i6 

Aluminum  (annealed)  

I    I  J7 

I   8^2 

Platinum  (annealed)  

3efjs 

5  882 

Iron  (annealed)  

•j  «2C 

6  2«; 

Lead   (pressed)  

7  728 

12  05 

Mercury 

62  so 

Carbon  (graphite)  . 

/•  '  3 

2CQO 

ALLOYS. 

8.240 

I3.I6 

Copper  60  %,  Zinc  26  £,  Nickel  14  <. 
Platinum-Silver.    .  .            

Q  4  C 

1C    -ifi 

Platinum  67  %,  Silver  33  %. 
Man^an  in     

•  v-o 

_Q    _ 

Copper  84  £,  Nickel  12  £,  Manganese  4  %. 

LIQUIDS  AT  18°  C. 
Pure  Water  

18.7 

IN  OHMS. 

30.30 

Dilut.  Sulphuric  Acid   5  % 

1020                 ) 

More  than 

INSULATORS. 

Glass  at  20°  C  

I.9I           j" 
IN  MEGOHMS. 
«6  X   IO8       } 

1,000,000 

Glass  at  200°  C  

g 

More  than 

Guttapercha  at  24°  C  

177  X  io«     \ 

1,000,000,000 

*  See  S.  P.  Thompson,  Elementary  Elec.  and  Mag.,  p.  404.      Also 
Electrical  Dictionary,  Houston. 


224  TESTING    OF    DYNAMOS    AND    MOTORS. 

Now5  to  build  up  this  formula:  If  the  conductor  is  i 
inch  long,  i  square  inch  cross-section,  and  we  deal  with 
pure  copper  whose  specific  resistance  is  i,  the  formula 
becomes: 

R  —  —  X  i  X  .00000068  =  .00000068  ohm, 

which  is  the  original  expression  for  the  resistance  of  a 
cubic  inch  of  pure  copper.  If  the  cube  is  L  times  as  long, 
the  cross-section  being  kept  the  same,  the  resistance 
becomes  L  times  as  great  and  we  have  R  =  L  X 
.00000068  ohm.  If  the  cross-section  becomes  A  times  as 
great,  R  is  diminished  as  many  times  as  A  will  go  into 
the  original  value  and  we  now  get 

_  L  X  .00000068 
R--  —j- 

If  we  go  further  and  use  a  metal  whose  specific  resistance  is 
not  i,  but  is  S,  the  formula  becomes  in  its  general  form, 


X  .00000068  ,    L 

R  =  ~  -  =  £  --X  .00000068  ohm. 

A  A 

Or  in  words,  the  resistance  of  any  conductor  varies  directly 
as  the  length,  directly  as  the  specific  resistance,  and  inversely  as 
the  cross-section  ;  and  = 

-^  X  68  X  io-8. 


As  an  example  of  the  formula's  usefulness,  we  will 
apply  it  to  the  designing  of  a  water  box,  which  shall  carry 
50  amperes  at  500  volts.  To  keep  the  boiling  and 
evaporation  within  proper  limits,  the  box  should  not 
carry  more  than  1/2  ampere  per  square  inch  cross- 


MEASUREMENT    OF    INSULATION.  225 

section.      Fifty  amperes  will  therefore  require  100  square 
inches  cross-section.      Now  since 

F  t^oo 

/  =  —  or  50  =     °R --,  R  --  10  ohms, 

which  must  be  the   resistance  of  the  box.     We  therefore 

have, 

A'  —         x  S  X  .00000068  ohm  or  L  —  —= —  --. 

A  6  X  .00000068 

Knowing  5  for  any  liquid,  L  is  readily  determined. 

In  closing  the  discussion  of  current,  E.  M.  F.,  and 
resistance  measurements,  we  will  consider  in  a  general 
way  what  instruments  are  best  suited  to  different  classes 
of  work.  A  high  resistance  Thomson  reflecting  gal- 
vanometer, provided  with  a  shunt  box  and  directing  mag- 
net, can  be  adapted  to  any  sort  of  work,  not  excepting 
ballistic  work,  if  the  needle  has  a  damping  vane  at- 
tached. The  Thomson  galvanometer  is,  however,  such 
an  expensive  and  delicate  instrument,  that  its  use  should 
be  confined  to  accurate  work,  and  a  lower  grade  article 
used  where  it  is  possible.  In  current  measurement, 
where  the  current  passes  through  the  galvanometer,  the 
latter  should  be  of  low  resistance.  To  use  a  high  resist- 
ance instrument  the  main  current  passes  through  a  shunt, 
and  the  instrument  takes  a  known  fraction  of  it.  In 
measuring  a  potential  difference  across  a  resistance,  the 
galvanometer  resistance  must  be  high  compared  to  that 
across  which  it  is  placed,  so  as  not  to  appreciably  lower 
the  resistance  between  the  two  points  to  which  its  ter- 
minals are  attached.  In  Fig.  85  suppose  R  to  be  an  un- 
known resistance  in  series  with  battery,  B.  A  is  an 
ammeter  and  G  a  galvanometer,  to  read  the  potential 


226  TESTING    OF    DYNAMOS    AND    MOTORS. 

difference  between  A  and  C.  If  G  is  of  low  resistance, 
placing  it  in  multiple  with  R  will  lower  the  resist- 
ance between  A  and  C,  and  G's  reading  will  be  valueless 

because  it  is   not  due  to 

I  f  I  I  P        r~^~|        nn  the    drop    on   R   alone. 

The  greater  R  compared 
with   G,  the  greater  the 
error   caused    by   intro- 
FIG.  85.  during  G.     Thus  a  gal- 

vanometer of  5,000  ohms 

resistance  would  be  a'comparative  short  circuit  across  a 
resistance  of  200,000  ohms,  and  hence  not  suitable  in 
such  a  case.  We  can  draw  the  conclusion,  then,  that  a 
high  resistant  galvanometer  is  adapted  to  reading  poten- 
tial differences  across  a  low  resistance  and  that  a  low  re- 
sistance galvanometer  is  not  adapted  to  reading  potential 
differences  at  all.  Again,  the  high  resistance  galvanom- 
eter must  be  used  if  great  sensibility  is  required.  A 
low  resistance  instrument  has  only  a  few  turns  of  wire 
on  it,  and  hence  produces  a  comparatively  weak  magnetic 
field;  it  is  more  useful  in  qualitative  than  in  quantitative 
work,  hence  its  use  in  the  Wheatstone  bridge  where  it 
merely  indicates  the  polarity  of  a  deflection;  the  same 
is  true  of  any  zero  method,  although  in  the  most  accurate 
zero  tests  the  high  grade  instrument  is  frequently  used. 

When  a  galvanometer  is  to  be  put  in  the  main  circuit 
the  character  of  instrument  is  determined  by  that  of  the 
circuit.  On  a  high  resistance  circuit  such  as  one  includ- 
ing a  piece  of  insulation  under  test,  although  the  galva- 
nometer may  be  of  high  resistance  it  will  not  materially 
affect  the  result,  for  its  resistance  is  small  when  com- 
pared to  that  of  the  insulation;  also  the  greater  sensitive- 


MEASUREMENT    OF    INSULATION.  227 

ness  of  the  instrument  is  an  advantage  in  this  work.  For 
series  use  in  a  low  resistance  circuit  the  low  resistance 
galvanometer  or  an  ammeter  is  to  be  used.  On  power 
and  lighting  circuits  the  instruments  are  permanently 
connected;  ammeters  should  be  of  as  low,  and  voltmeters 
of  as  high  resistance  as  possible,  so  as  to  consume  only 
the  smallest  amount  of  energy.  Ammeters  are  but  low 
resistance  galvanometers;  voltmeters  are  high  resistance 
galvanometers.  The  ammeter's  resistance  is  made  low 
so  that  the  drop  through  it  shall  be  small.  The  volt- 
meter's resistance  is  made  high  so  that  it  shall  take  but  a 
small  current.  A  station  full  of  poorly  designed  meters 
is  a  source  of  considerable  loss. 

The  broad  and  general  rule  for  all  measuring  instru- 
ments is  that  they  must  be  such  as  to  in  no  way  disturb, 
or  modify  the  existing  conditions  of  the  circuit  in  which 
they  are  to  be  introduced  and  used. 


TESTING  OF  DYNAMOS  AND  MOTORS 


TESTING  OF  DYNAMOS  AND  MOTORS. 
CHAPTER  VIII. 

THE    SERIES    MACHINE. 

THE  simplest  type  of  self-exciting  dynamo  is  the 
series  wound,  a  diagram  of  which  is  given  in  Fig.  86.  A 
is  the  armature,  F  the  field  coils,  and  L  L  lamps  or  other 
load.  The  distinguishing  characteristic  of  this  type  is 
the  interdependence  of  all  parts  of  the  circuit.  There  is 
but  one  circuit,  and  breaking  any  part  of  it  whatever 
throws  the  machine  out  of  action.  As  a  motor,  it  has 
several  features  which  make  it  very  valuable  in  street 
railway,  or  other  work  where  frequent  interruptions 
of  the  circuit  are  incidental  to  the  service.  In  doing 
heavy  duty,  suddenly  imposed,  there  flows  through  the 
circuit  an  abnormal  current,  which  if  prolonged  might 
do  harm,  but  since  the  field  current  grows  at  the  same 
rate  as  that  of  the  armature,  there  develops  a  powerful 
C.  E.  M.  F.,  which  checks  the  rise  of  the  current.  This 
and  other  features,  combined  with  the  fact  that  there 
is  but  one  circuit  to  control,  makes  the  series  motor  an 
ideal  one  for  street-car  work. 

The  series  dynamo  is  adapted  to  arc  lamp,  or  other 
service  where  a  constant  current  must  be  maintained 
through  a  variable  external  resistance.  The  field  winding 

331 


232  TESTING    OF    DYNAMOS    AND    MOTORS. 

being  included  in  the  main  circuit,  the  armature  does  not 
generate  until  the  circuit  is  closed,  and  then  the  readiness 
with  which  it  does  so  depends  upon  the  external  resist- 
ance, the  speed,  and  in  lesser  measure  upon  the  strength 
of  the  residual  field,  and  the  re- 
luctance of  the  magnetic  circuit. 
For  a  given  external    resistance 
the  initial  flow  of  current  will  de- 
pend upon  the  speed  and  residual 
field:   if  the  reluctance  is  small, 
the  field  will   respond  promptly, 
and   in  any  case  builds   up  until 
the  current  reaches   such  a  value 

that  the  internal  "drop"  balances  the  gain  in  E.  M. 
F.  due  to  increase  of  field.  As  the  speed  rises  so 
does  the  E.  M.  F. ,  and  with  it  the  current,  hence  also  the 
field  strength:  finally  the  field  cores  become  saturated, 
being  usually  of  lower  permeability  than  the  armature, 
and  above  this  point  the  field  strength  remains  practically 
constant.  For  every  particular  value  of  external  resist- 
ance there  is  a  speed  corresponding  to  field  saturation, 
and  below  this  speed  the  machine  will  not  work  satisfac- 
torily; this  is  called  the  critical  speed,  and  below  it  the 
field  is  too  light,  fluctuating  with  every  slight  variation  of 
load.  So,  also,  to  every  speed  there  corresponds  a  criti- 
cal resistance  above  which  the  current  is  too  low  to  suffi- 
ciently excite  the  fields.  The  higher  the  speed,  the 
higher  the  resistance  through  which  a  series  dynamo  will 
excite  itself,  and  conversely,  the  lower  the  resistance,  the 
lower  the  critical  sj^eed.  A  point  to  be  noted  in  connec- 
tion with  the  critical  speed  is,  that  a  series  dynamo  will 
maintain  an  established  field  with  a  smaller  current  than 


THE    SERIES    MACHINE.  233 

it  takes  to  produce  that  field.  Thus,  if  a  high  resistance 
be  hi  circuit  while  the  armature  runs  at  constant  speed, 
and  the  resistance  be  decreased  till  the  field  "  picks  up," 
the  machine  will  hold  its  field  even  when  the  resistance  is 
again  increased:  however,  when  the  field  once  begins  to 
fall  it  does  so  very  rapidly,  a  process  assisted  by  vibra- 
tion of  floor  and  machine.  On  the  other  hand  the  field 
builds  up  with  equal  rapidity,  and  in  putting  on  a  load 
care  must  be  had  lest  an  excessive  P>.  M.  F.  be  developed 
and  trouble  ensue.  This  point  is  well  illustrated  in  a  bar 
commutator  arc  light  machine.  When  working,  each  arc 
has  a  C.  K.  M.  F.  of  about  50  volts,  which  is  absent  when 
the  load  is  first  thrown  on;  through  the  very  low  re- 
sistance at  start  the  initial  E.  M.  F.  sends  a  large  current, 
over  saturating  the  fields,  with  a  twofold  result:  first 
the  neutral  line  is  advanced,  and  if  the  brushes  are  not 
quickly  brought  forward,  vicious  sparking  results;  sec- 
ond, there  is  an  excessive  E.  M.  F.,  which,  availing  itself  of 
the  sparking,  may  flash  around  the  commutator.  Spark- 
ing assists  the  flashing  by  volatilizing  the  copper  or  car- 
bon surfaces,  thereby  providing  a  conducting  vapor  for 
the  arc.  This  flashing,  so  common  to  arc  machines,  can 
be  caused  by  a  sudden  variation  of  external  resistance. 
On  full  load  the  voltage  may  be  2,500  or  3,000;  if  the 
line  be  now  broken,  the  neutral  point  is  thrown  back 
away  from  the  brushes  and  sparking  is  set  up,  resulting, 
generally,  in  flashing  from  brush  to  brush.  Fortunately 
the  arc  machine  has  an  inherent  factor  of  safety  in  its 
armature  reaction,  assisted  by  comparatively  easily  sat- 
urated  field  cores.  If  the  speed  be  kept  constant,  at 
some  value  above  the  critical  value,  and  the  resistance  be 
gradually  decreased,  the  current  will  rise,  but  the  field 


234  TESTING    OF    DYNAMOS    AND    MOTORS. 

will  remain  almost  the  same,  for  after  saturation 
the  magnetism  does  not  respond  so  readily  to  variations 
of  current.  As  already  learned,  the  internal  drop  increases 
with  the  current,  and  hence  the  terminal  E.  M.  F.  de- 
creases: furthermore  shifting  the  brushes  forward  to 
meet  the  requirements  of  the  neutral  line,  brings  like 
poles  of  armature  and  field  closer  together,  and  places  the 
former  in  a  position  to  exercise  greater  demagnetizing 
influence  on  the  latter,  thereby  weakening  the  field  and 
reducing  the  E.  M.  F.  so  much  that  at  over  load  it  is  less 
than  at  quarter,  or  half  load,  when  the  reaction  is  not 
so  great.  Every  self-exciting  dynamo  should  be  able  to 
pick  up  its  field  readily  when  the  armature  runs  at  its 
rated  speed:  and  in  the  case  of  the  series  dynamo  with 
the  added  condition  that  the  circuit  resistance  is  not  too 
high.  In  a  series  dynamo  failure  to  pick  up  indicates  a 
fault  in  either  the  machine  or  the  line.  Whether  it  is 
internal  or  external  can  be  determined  by  holding  a 
short  wire  across  the  terminals  of  the  line,  so  as  to  cut 
out  all  resistance  save  that  of  armature  and  field  coils. 
If  the  machine  now  generates,  the  fault  is  in  the  line; 
otherwise,  it  is  internal. 

Among  the  more  common  troubles  occurring  to  self- 
exciting  dynamos  are  the  following:  (i)  Loss  of 
residual  field;  (2)  wrong  connection  of  armature  or 
field;  (3)  open,  or  (4)  short  circuit  in  armature  or  field; 
(5)  high  resistance  of  brush  contact,  due  to  oxidation 
of  brushes,  or  to  shellac  on  the  commutator;  (6)  the 
speed  may  be  below  the  critical  value.  To  these  may  be 
added  as  less  common:  (7)  loosening  of  a  field  core  or 
pole  piece;  (8)  armature  core  below  standard  diameter; 
(9)  pole  pieces  bored  above  standard  diameter.  The 


THE    SERIES    MACHINE.  235 

effect  of  the  last  three  failings  is  to  increase  the  magnetic 
reluctance,  thereby  reducing  the  E.  M.  F.  of  the  machine 
and  impairing  its  ability  to  pick  up  a  field.  As  a  motor 
the  C.  E.  M.  F.  would  be  reduced  and  the  last  three  faults 
would  cause  an  increase  in  speed.  On  street  railway 
motors  parts  frequently  work  loose  as  a  result  of  pounding. 

The  best  order  to  be  observed  in  testing  for  faults 
depends  upon  the  circumstances  of  each  particular  case, 
so  only  general  directions  can  be  given  here.  When 
a  machine  in  active  service  suddenly  refuses  to  gener- 
ate, the  trouble  may  be  due  to  a  burn  out,  to  loss 
of  residual  field,  to  open  field  circuit;  or  on  a  series 
machine  to  some  interruption  on  the  line,  e.  g.,  a  loose 
lamp  connection  or  a  wire  broken  by  swinging  in  the 
wind  but  held  up  by  the  insulation.  If  a  burn  out 
occurs  while  the  machine  runs  the  symptoms  will  leave 
little  doubt  as  to  the  cause,  but  experience  shows  that  as 
many  machioes  burn  out  stan'ding  still,  as  do  running. 

Recharging  will  restore  a  lost  residual  field.  Generally, 
in  course  of  construction,  the  fields  acquire  under  the  file 
and  hammer  sufficient  magnetism  to  obviate  the  neces- 
sity of  charging,  but  this  is  not  always  the  case,  and 
strange  to  say,  machines  in  service  from  day  to  day  have 
been  known  to  not  only  lose  their  residual  field,  but  to 
even  acquire  one  of  reversed  polarity.  Since  it  is  impor- 
tant that  a  machine's  polarity  should  remain  always  the 
same,  recharging  is  resorted  to  when  necessary.  This  is 
done  by  connecting  the  fields  in  series  with  a  machine 
known  to  be  in  working  order.  When  the  field  or  arma- 
ture terminals  have  been  disturbed  in  any  way,  and 
the  machine  will  not  generate,  either  set  of  terminals,  but 
not  both,  should  be  reversed  if  there  is  any  uncertainty 


236  TESTING    OF    DYNAMOS    AND    MOTORS. 

as  to  connections.  It  is  easy  to  confuse  connections  in 
reconnecting  a  machine  dismounted  for  repair,  and  unless 
the  proper  relation  exists  between  armature  and  field  con- 
nections, the  machine  will  not  generate :  for  example,  sup- 
pose the  two  halves  of  the  field  winding  to  be  bucking 
each  other:  there  will  result  no  field  at  all,  for  any 
tendency  which  one  spool  exerts  toward  a  definite 
polarity  is  neutralized  by  the  counter  tendency  of  the 
second  spool.  For  securing  proper  connection  every 
dynamo  has  its  rule  which  must  be  observed  to  obtain 
satisfaction.  A  rule  familiar  to  those  experienced  with 
Edison  machines  is  as  follows:  "  Connect  the  right-hand 
magnet  to  the  lower  brush,"  for  the  shunt  winding,  and 
the  "right-hand  magnet  to  the  upper  brush,"  for  the 
series.  The  difference  is  due  to  the  fact  that  on  one 
winding  the  inside  ends,  on  the  other  the  outside  ends, 
are  brought  out  for  connection.  Where  no  rule  is  given, 
and  former  experience  gives  no  clew,  direct  experiment 
is  resorted  to,  and  the  connections  shifted  till  the  right 
combination  i"s  secured.  The  proper  shunt  connections 
once  secured,  it  is  easy  to  get  the  series  connections  right, 
as  will  be  shown  under  generator  testing. 

It  sometimes  happens  that  a  dynamo  when  first  started 
shows  a  small  E.  M.  F.  due  to  the  residual  field,  but  on 
closing  the  field  circuit  the  E.  M.  F.  falls  to  zero,  and  the 
machine  refuses  to  generate  at  all.  Such  action  indicates 
a  wrong  connection  of  field  or  armature,  and  can  be  ex- 
plained as  follows:  Suppose  the  dynamo  to  be  properly 
connected  and  to  be  generating;  this  involves  the  follow- 
ing conditions:  that  the  field  current  is  in  such  a  direc- 
tion as  to  produce  a  magnetic  field  which  shall  enable  the 
armature  conductors,  cutting  this  field,  to  generate  an 


THE    SERIES    MACHINE.  237 

E.  M.  F.  that  in  turn  shall  reinforce  the  initial  arma- 
ture current  due  to  the  residual  field.  Now,  without 
disturbing  anything  else,  let  the  field  terminals  be  re- 
versed. For  the  sake  of  clearness  we  will  suppose 
that  there  remains  a  residual  fiekl,  due  to  the  current  last 
flowing:  under  this  condition  the  pole  pieces  are  of  the 
same  polarity  as  when  the  machine  was  properly  con- 
nected, and  was  generating:  since  the  lines  of  force  due 
to  the  residual  field  are  in  the  same  direction  as  when  the 
armature  generated.  The  small  current  now  in  the  arma- 
ture, and  due  to  the  residual  field,  will  be  in  the  same 
direction  as  it  was  then,  but  the  field  connections  being 
reversed,  the  current  now  flows  around  the  spools  in 
such  a  direction  as  to  neutralize  the  residual  magnet- 
ism. The  slight  magnetizing  force  due  to  the  residual 
field  now  opposes  this  field  and  soon  reduces  it  to  zero, 
thus  totally  depriving  the  machine  of  all  ability  to  pick 
up.  Nor  can  a  reverse  field,  even  if  established  by  re- 
charging, be  maintained;  for  assuming  a  reversed  residual 
to  be  provided,  the  lines  of  force  have  changed  direction, 
the  armature  current  does  likewise,  and  previous  con- 
ditions being  re-established  the  residual  field  is  again 
destroyed.  Confusion  of  connections  is  a  common  source 
of  failure  to  generate,  but  there  are  other  causes  less 
common  but  similar  in  effect.  Among  these  are  errors 
in  winding  and  connecting.  In  rewinding  an  armature 
the  new  coils  should  be  wound  on  in  the  same  way  that  the 
old  ones  were.  Other  things  being  the  same,  the  armature 
polarity  depends  upon  the  manner  of  winding  and  con- 
necting. If  in  the  original  armature,  the  winder  turned 
the  core  over  and  from  himself,  in  rewinding  the  same 
rule  must  be  observed.  In  connecting,  the  leads  may  be 


238  TESTING    OF    DYNAMOS    AND    MOTORS. 

brought  around  oppositely  to  what  they  were  originally 
or  they  may  be  brought  around  one  bar  too  far,  any  of 
which  mistakes  reverses  the  armature  polarity,  and 
changes  its  relation  to  the  field.  Under  these  circum- 
stances, the  attendant  may  be  very  much  surprised  to 
find  that  his  dynamo  refuses  to  generate  under  apparently 
the  same  conditions  that  existed  before  rewinding.  In 
such  a  case,  a  reversal  either  of  direction  of  rotation,  or  of 
field  or  arntature  terminals,  will  restore  the  dynamo  to 
working  order.  It  is  not  always  convenient  to  reverse 
the  direction  of  rotation,  as  it  involves  either  reversing 
the  engine  or  turning  the  dynamo  end  for  end;  while  the 
brushes,  if  of  copper,  must  also  be  changed,  and  this  may 
necessitate  altering  the  brush  holders.  On  arc  machines 
provided  with  regulating  devices  the  latter  are  often  con- 
structed and  adjusted  for  a  given  direction  of  rotation. 
On  such  a  machine  let  us  suppose  the  regulation  to  be 
effected  by  means  of  the  brushes,  and  also  suppose  the 
direction  of  rotation  to  be  changed.  If  for  increase  of 
current  the  mechanism  throws  the  brushes/tfr^w^/giving 
them  a  positive  lead,  with  the  reversed  rotation,  this 
would  now  be  a  negative  lead  instead  of  positive;  while 
for  decreasing  current  drawing  the  brushes  back  is  to  give 
them  a  positive  lead.  In  either  case  the  sparking  is 
increased  instead  of  diminished,  and  in  the  latter  case 
flashing  is  apt  to  ensue.  It  is  therefore  necessary  to 
preserve  the  direction  of  rotation  undisturbed. 

If  one  of  the  above  reversals  is  found  necessary,  it  is 
customary  to  reverse  the  field  connections,  as  they  are 
near  together  and  a  neat  job  can  be  done.  Having 
determined  that  failure  to  generate  is  not  due  to  wrong 
connection  some  other  source  of  trouble  must  be  sought 


THE    SERIES    MACHINE.  239 

There  are  many  tests  for  open  and  short  circuit  in  field 
or  armature.  If  while  the  fields  are  separately  excited, 
the  armature  be  rotated,  a  short  circuit  in  the  fields  will 
probably  be  indicated  by  too  low  voltage  at  the  brushes, 
since  part  of  the  field  coil  is  inactive,  and  the  armature 
is  unable  to  produce  the  voltage  called  for  by  the 
known  voltage  applied  to  the  fields.  This  test  is  not 
infallible,  and  an  existing  short  circuit  might  escape 
detection,  unless  the  field  circuit  contains  an  ammeter, 
and  the  field  current  is  maintained  at  normal  value. 
Thus,  suppose  there  are  two  field  coils  of  equal  mag- 
netizing power  and  resistance,  and  that  the  short  circuit 
is  such  as  to  entirely  cut  out  one  coil;  for  a  given 
E.  M.  F.  at  the  field  terminals,  the  current  will  now  be 
twice  its  normal  value,  and  hence  the  field  ampere-turns 
exactly  the  same  as  if  no  short  circuit  existed :  the  field 
strength  will  therefore  be  unaltered.  If  the  fields  under 
test  are  those  of  a  constant  current  machine,  and  are 
connected  in  series  with  a  second  constant  current 
machine,  or  in  any  case  if  the  field  current  is  kept  con- 
stant, the  test  becomes  a  decisive  one.  Open  circuit 
would,  in  a  series  machine,  be  indicated  by  the  absence 
of  all  current.  Open  circuit  in  the  fields  can  in  all  cases 
be  tested  for  with  an  ordinary  bell  and  battery,  a  mag- , 
neto,  or  a  test  lamp  circuit.  Fig.  87  gives  the  connections 
for  a  bell  and  battery.  B  is  the  battery  of  two  cells, 
and  at  M  a  bell.  T  T'  are  test  lines.  The  whole  outfit 
can  be  put  into  a  small  box,  and  T  T',  when  not  in  use, 
are  coiled  around  a  cleat  on  the  cover.  Fig.  88  gives 
the  arrangement  of  a  test  lamp  circuit.  A  and  B  are 
light  or  power  mains,  Ll  Z2  Z3  are  lamps  in  series,  the 
number  depending  upon  the  voltage  in  use:  these  lamps 


240 


TESTING    OF    DYNAMOS    AND    MOTORS. 


are  inserted  in  keyless  sockets  mounted  on  a  board. 
T  71',  are  test  lines,  and  just  before  use  should  be  held 
together  to  insure  that  the  lamp  circuit  is  intact:  they 
are  then  held  across  the  field  terminals,  when  the  light- 
ing of  the  lamps  will  indicate  the  fields  to  be  a  closed 


5 

V-^\> 

r  . 

T 
FIG.  8; 

circuit.  If  the  test  circuit  is  taken  from  a  street  rail- 
way line,  one  must  bear  in  mind  that  one  side  of  it  is 
''grounded,"  and  should  the  machine  under  test  be  on 
the  ground,  some  of  the  results  might  be  delusive.  If  the 
normal  field  resistance  is  known,  short  or  open  circuit 
can  be  readily  detected  by  measuring  it;  if  it  proves  to 
be  high  or  infinitely  great,  a  partial  or  complete  open 
circuit  exists;  while  a  low  value  indicates  short  circuit. 
If  the  fields  are  in  sections  or  on  separate  spools,  they 
should  be  disconnected,  tested  individually,  and  the 
faulty  one  removed.  In  cases  of  necessity,  and  where  a 
machine  runs  alone,  the  faulty  spool  can  be  disconnected 
and  left  till  a  more  convenient  time  for  repairing  it. 

This  test  can  be  applied  to  armatures  when  their  brush 
to  brush  or  bar  to  bar  resistance  is  known.  In  general 
it  is  more  difficult  to  locate  armature  troubles,  unless  gal- 
vanometer tests  are  resorted  to,  because  there  is,  through 
the  armature,  a  double  path  fromibrush  to  brush,  so  that 


THE    SERIES    MACHINE.  24! 

although  a  wire  may  be  broken  in  one  path,  the  circuit  is 
complete  through  the  other.  If  the  armature  be  run  in  sep- 
arately excited  fields  it  will  spark  at  the  brushes  in  case  of 
either  an  open  or  short  circuit,  because  the  armature  is 
electrically  unbalanced,  and  the  neutral  point  shifts  con- 
stantly. A  short  circuit  can  often  be  located  by  remov- 
ing the  brushes,  and  running  the  armature  at  its  usual 
speed  in  the  fully  excited  field.  The  field  must  be 
separately  excited,  for  with  a  defective  armature  the 
machine  will  not  excite  itself  even  though  the  brushes  be 
restored.  Now  the  resistance  of  a  whole  armature  is 
not  high,  and  the  resistance  of  the  short  circuited  coil 
being  very  low,  the  electromotive  force  generated  by  the 
coil  produces  sufficient  current  to  heat  the  wire  and 
ultimately  to  burn  it  out.  Open  or  short  circuit  can  also 
be  detected  by  separately  exciting,  as  above,  then  turn- 
ing the  armature  slowly,  and  watching  the  needle  of  a 
voltmeter  placed  across  the  brushes.  In  case  of  open 
circuit,  the  needle  will  fluctuate  between  zero  and  some 
definite  value,  depending  upon  the  field  strength  and  the 
speed.  If  a  short  circuit  is  present,  the  needle  will  not 
drop  to  zero,  but  will  fluctuate  between  two  definite 
values.  The  above  methods  were  well  illustrated  in 
the  following  case,  of  an  armature  which  refused  to  move 
when  connected  as  a  motor;  it  was  then  run  as  a  sepa- 
rately excited  generator,  and  the  voltmeter  read  zero 
across  the  brushes,  but  the  armature  became  very  warm 
after  a  ten-minute  run.  The  heating,  however,  was  not 
local,  but  uniform  over  the  whole  armature.  The  com- 
mutator was  then  disconnected  and  tested,  and  the 
insulation  between  bars  found  to  be  defective,  thus  short 
circuiting  every  coil  upon  itself.  Connected  as  a  motor 


OF  THE 

((UNIVERSITY  J 


242 


TESTING    OF    DYNAMOS    AND    MOTORS. 


the  current  entering  at  one  brush  passed  directly  through 
the  carbonized  insulation  and  the  commutator  bars  to  the 
other  brush,  without  entering  the  coils  at  all;  when  run 
as  a  generator,  each  coil,  short  circuited  on  itself,  was 
heated,  but  since  there  was  no  E.  M.  F.  additive  from 
brush  to  brush  the  voltmeter  gave  no  sign. 

The  same  method  in  a  modified  form  can  be  used  to 
detect  a  ground  in  an  armature,  the  effectiveness  of  the 
test  depending  upon  the  fact  that 
in  a  defective  armature  the  E. 
M.  F.  generated  is  greater  when 
the  faulty  coil  is  in  one  part  of  the 
field  than  when  in  another.  The 
voltmeter  will  indicate  this  fluctua- 
tion if  the  speed  is  not  too  high. 
When  an  armature  becomes  inter- 
nally grounded  it  is  customary 
'to  burn  out  the  fault  if  possible 
so  that  it  may  be  more  readily 
seen  in  course  of  stripping;  this  is  a  practice  adhered 
to  even  in  the  case  of  a  short  circuit,  but  the  writers 
do  not  approve  of  the  practice  unqualifiedly  in  that 
it  often-  proves  to  be  an  injustice  to  the  winder. 
Apparent  difficulty  in  an  armature  can  often  be  traced 
to  a  defect  in  the  commutator,  by  disconnecting  and 
testing  it,  and  such  a  fault  can  certainly  not  be  laid 
on  the  winding.  Fig.  89  gives  connections  for  detecting 
and  burning  out  a  ground.  The  armature,  A,  is  grounded 
at  G'  (/.  e.,  the  copper  wire  touches  the  iron  core,  or  as 
railway  men  say :  "The  machine  has  an  iron  circuit "). 
A  second  ground  is  created  by  connecting  one  brush  to 
the  frame.  If  the  machine  be  now  run  as  a  separately 


FIG.  89. 


THE    SERIES    MACHINE.  243 

excited  generator  and  from  a  motor  of  moderate  capacity, 
the  armature  will  alternately  start  and  stop  once  in  every 
revolution.  This  "bucking"  is  not  so  marked  if  the 
driving  power  is  a  larger  unit.  When  the  fault,  G',  is  in 
the  position  of  the  figure,  there  are  two  short  circuits 
present,  one  through  G ',  i,  E,  4,  6,  G,  the  other  through 
£',  2,  E,  4,  6,  G.  Two  parts  of  the  winding  are  in  mul- 
tiple, and  through  the  low  resistance  of  the  short  circuit 
a  large  current  flows,  loading  the 
dynamo  so  that  the  motor  cannot 
run  it.  As  soon  as  the  dynamo 
stops,  its  current,  and  hence  load, 
becomes  zero,  and  the  motor  thus 
relieved  starts  again,  only  to  re- 
peat the  same  operation.  The 
point  of  maximum  speed  corre- 
sponds to  the  position  where 
the  fault  passes  under  the 
grounded  brush  holder,  for  it  is 

here  the  two  grounds  become  one,  and  have  no  more 
influence  than  a  single  ground  has  on  any  otherwise 
insulated  circuit.  As  G'  is  carried  past  the  grounded 
brush  the  E.  M.  F.  in  the  grounded  circuit  rises  and  cur- 
rent once  more  flows.  Fig.  90  shows  a  method  of  find- 
ing open  circuit:  Suppose  a  break  to  be  at  A;  it  is 
desired  to  determine  between  what  two  bars  it  is  located. 
Open  a  circuit  at  a  second  point,  as  at  B,  by  disconnect- 
ing one  of  the  leads.  There  are  now  two  parts  of  the 
winding  insulated  from  each  other.  Next  place  one  test 
line  on  bar  No.  i  and  move  the  other  around  the  commu- 
tator till  the  bell  ceases  to  ring,  which  will  be  as  soon  as 
the  sliding  contact  rests  on  bar  No.  10,  thus  showing  the 


244  TESTING    OF    DYNAMOS    AND    MOTORS. 

fault  to  be  between  Nos.  9  and  10.  To  verify  the  result, 
place  one  line  on  No.  n  and  slide  the  other  back  and 
forth  over  Nos.  9  and  10:  the  bell  will  cease  ringing  upon 
reaching  bar  9.  Enough  of  the  head  can  now  be  removed 
to  examine  the  damaged  coil.  Trouble  of  this  nature 

does  not  generally  occur  in 
the  more  removed  parts  of 
the  winding,  and  is  often  due 
to  the  melting  of  a  connect- 
ing wire.  Poorly  "  sweated  " 
connections  are  a  fruitful 
source  of  trouble:  a  poor 
connection  will  always  heat 
and  may  reach  a  temperature 
sufficient  to  melt  the  solder  and  release  the  connector 
from  the  ear  or  cup  of  the  commutator  bar,  thereby 
causing  an  open  circuit.  Such  trouble  is  more  often  due 
to  mismanagement  than  to  any  defect  in  the  machine 
itself. 

We  have  learned  that  a  short  circuit  in  the  field  wind- 
ing impairs  a  machine's  ability  to  excite  itself  and  in 
increased  measure  as  the  short  circuited  portion  is 
greater.  This  fact  is  utilized  on  series  dynamos  for 
removing  them  from  service.  In  Fig.  91  A'  is  the  line 
switch  and  K'  one  which  short  circuits  the  field,  F. 
When  the  dynamo  is  in  service  K  is  closed  and  K1  open. 
To  remove  the  load  K'  is  closed.  K  is  generally  left 
closed. 

Whether  failure  to  generate  is  or  is  not  due  to  low 
speed  can  be  ascertained  by  taking  the  speed  of  the 
armature  or  that  of  the  countershaft  which  drives  it,  and 
multiplying  this  by  the  ratio  of  the  diameter  of  the 


THE    SERIES    MACHINE.  245 

countershaft  pulley  to  that  of  the  armature  pulley. 
Assuming  that  the  engine  and  countershaft  speed  are 
correct,  that  of  the  armature  is  governed  by  the  size  of 
its  pulley,  and  can  be  altered  by  using  a  pulley  of  greater 
or  less  diameter;  this  step,  however,  should  not  be  taken 
unless  the  designed  speed  of  the  armature  is  known,  for 
many  armatures  will  not  run  at  full  load  without  sparking, 
if  the  speed  be  much  below  or  much  above  its  rated  value. 
As  a  rule,  in  order  to  secure  sparkless  running  the  field 
magnets  are  made  so  powerful,  and  the  shunt  and  series 
coils  (on  a  compound-wound  machine)  so  proportioned  that 
even  on  full  load  the  armature  causes  but  slight  distortion 
of  the  field,  and  the  position  of  the  neutral  points,  hence  of 
the  brushes,  is  practically  unaltered.  In  poorly  designed 
constant  potential  machines,  and  in  series  machines  in- 
tended to  regulate  for  constant  current,  this  proportioning 
of  armature  and  field  is  not  observed,  so  that  the  position  of 
the  neutral  points  depends  upon  the  load;  changing  the 
load  shifts  the  position  of  the  neutral  points,  and  unless 
the  brushes  are  shifted  accordingly,  sparking  ensues. 
The  effect  of  changing  the  armature  speed  is,  for  any 
given  load,  to  change  the  above  relation  between  armature 
and  field,  and  hence  to  cause  a  shifting  of  the  neutral  line, 
with  resulting  sparking.  For  a  given  position  of  the 
brushes  sparking  can  be  avoided  only  by  strengthening 
or  weakening  the  field  enough  to  restore  the  neutral  line 
to  its  position  under  the  brushes.  This  is  an  effect  seen 
not  only  on  badly  designed  constant  potential  machines, 
but  also  on  well  designed  ones  if  the  armature  is  run 
much  below  its  rated  speed.  To  have  the  field  strong 
enough  to  control  the  position  of  the  neutral  point  there 
must  be  the  designed  voltage  at  the  field  circuit  terminals: 


246  TESTING    OF    DYNAMOS    AND    MOTORS. 

to  lower  the  speed  is  to  lower  this  voltage,  and  with  it 
the  field  current,  so  the  field  magnetizing  effect  is  lower, 
while  that  of  the  armature  remains  constant.  Raising 
the  speed  strengthens  the  field,  but  raises  the  E.  M.  F. 
of  each  coil  as  it  is  short  circuited  by  the  brush,  and 
enables  it  to  generate  current  enough  to  cause  sparking. 
This  is  avoided  in  ordinary  running  by  using  carbon 
brushes  of  comparatively  high  resistance. 

Series  machines  are  widely  used  on  arc  light  circuits, 
and  when  run  together  are  generally  connected  in  series. 
When  so  connected  they  give  no  trouble-in  load  regula- 
tion, each  unit  supplying  its  share  of  the  total  voltage, 
the  same  current  passing  through  all.  The  total  watts 
generated  equals  the  total  E.  M.  F.  X  current  =  E  I  = 
(e  -f-  ^,  +  ^2  +  ^3  +,  etc.)  7,  where  e,  elt  etc.,  are  the 
E.  M  Fs.  of  the  individual  machines,  or  W  =  (e  -\-  el  -\- 
ei  +  e3  -K  etc- )  ^»  anc*  we  see  that  the  load  on  each  machine 
is  directly  proportional  to  its  E.  M.  F.  Machines  of 
different  capacities  can  therefore  be  run  together  without 
overloading  any,  provided  /  does  not  exceed  the  current 
capacity  of  the  smallest  machine.  The  advantage  of  the 
series  connection  is  that  many  lamps  can  be  placed  on 
one  circuit,  thereby  saving  copper  and  other  expenses. 
Series  machines  can  be  run  in  multiple,  provided  one  of 
the  following  precautions  is  taken:  (i)  the  machines  with 
rigid  connection  between  the  shafts  (direct  coupled), 
must  be  started  up  with  both  line  switches  in;  or  (2)  the 
fields  must  be  separately  excited,  when  of  course  they  cease 
to  be  series  machines;  or  (3)  an  equalizer  must  be  used 
to  regulate  the  sharing  of  the  load,  and  to  prevent  any 
dynamo  from  reversing  and  running  as  a  motor.  The  equal- 
izer is  especially  used  with  compound-wound  machines, 


THE    SERIES    MACHINE.  247 

and  its  consideration  is  therefore  deferred.  If  the  arma- 
ture shafts  are  rigidly  connected  the  necessity  of  start- 
ing with  both  line  switches  in  is  easily  seen;  for  since 
a  series  machines  is  devoid  of  field  until  the  external 
circuit  is  closed,  throwing  in  its  line  switch,  when  other 
machines  are  in  service,  would  result  in  a  short  circuit 
through  its  armature.  This  would  run  the  machine  in 
question  as  a  motor  and  in  the  opposite  direction,  since 
series  motor  and  dynamo  reverse  rotation  for  given  con- 
nections. The  rigid  connection  would  prevent  actual 
reversal  of  rotation,  but  unless  a  clutch  slipped,  or  a  cir- 
cuit breaker  worked  very  promptly,  something  serious 
would  happen.  With  separate  excitation  the  machine's 
E.  M.  F.  can  be  made  equal  and  opposite  to  that  on  the 
line,  and  its  switch  closed.  Other  things  being  equal, 
the  load  which  each  machine  takes  depends  upon  its  own 
E.  M.  F.,  and  this  in  turn  upon  the  field  strength.  With 
separate  excitation  regulation  is  obtained  by  means  of 
a  rheostat,  and  automatic  devices  depending  upon  the 
armature  reaction  are  generally  dispensed  with.  In 
practice  the  only  circumstances  under  which  arc  machines 
are  run  in  multiple,  is  where  the  lamps  used  require  a 
current  double  that  of  either  machine.  The  machines 
cannot  be  expected  to  regulate  very  closely,  unless  they 
are  magnetically  and  electrically  balanced  throughout 
their  load  range.  As  a  rule  the  lamps  do  not  give 
satisfaction. 

Series  dynamos  can  be  designed  to  regulate  for  a 
fairly  constant  potential  between  certain  load  limits. 
We  have  learned  that  below  a  certain  current  value  the 
fields  are  insufficiently  excited,  and  up  to  this  point  the 
voltage  fluctuates.  If  this  critical  point  be  brought  near 


248  TESTING    OF    DYNAMOS    AND    MOTORS. 

the  saturation  point  of  the  iron,  the  voltage  due  to 
increased  field  strength  as  load  goes  on  may  increase 
at  the  same  rate  as  the  drop  in  the  line  increases,  thus 
giving  constant  potential  at  the  lamps  or  other  load 
across  the  mains.  The  limits  of  regulation,  however,  are 
narrow,  and  for  light  and  heavy  loads,  poor.  In  attempt- 
ing to  run  several  machines  in  multiple  there  arise  com- 
plications which  practically  exclude  series  dynamos  from 
this  class  of  work,  now  covered  by  shunt- and  compound- 
wound  dynamos.  On  the  other  hand,  while  arc  lamps 
are  generally  run  from  series  machines,  fhey  are  often 
operated  from  the  same  dynamo  that  supplies  incan- 
descent lamps,  or  trolley  system,  at  constant  potential. 
In  this  case  as  many  lamps  are  placed  in  series  as  can  be 
worked  from  the  given  voltage,  and  a  German  silver 
resistance  is  placed  in  circuit  with  them  to  avert  the 
short  circuit,  which  otherwise  would  occur  were  all  the 
carbons  to  come  together  at  once.  This  added  resist- 
ance also  improves  the  regulation  of  the  lamps,  which 
are  unsteady  when  connected  directly  across  the  mains. 
The  effect  of  the  resistance  is  to  cushion  the  current 
fluctuations  by  narrowing  the  limits  between  which  it 
can  fluctuate.  Thus  suppose  two  arc  lamps  are  in  series 
across  125  volt  mains.  Let  the  total  lamp  resistance 
when  the  carbons  are  together  be  i  ohm,  and  let  the 
extra  resistance  be  4  ohms,  making  the  total  circuit 
resistance  5  ohms.  Without  the  extra  resistance  the 
current  could  fluctuate  between  o  and  125  amperes,  while 
with  it  the  limits  would  be  o  and  25  amperes,  the  zero 
value  corresponding  to  open  circuit,  due  to  the  carbons 
drawing  too  far  apart.  On  modern  lamps  such  action  is 
hardly  probable,  as  each  lamp  is  provided  with  a  device 


THE    SERIES    MACHINE.  249 

for  cutting  out  the  lamp  when  the  arc  reaches  a  certain 
length. 

The  problem  of  regulation  on  arc  machines  is  solved 
in  one  of  three  ways.  In  the  first  there  is  a  mechanism 
operated  either  by  a  solenoid  around  which  the  working 
current  flows,  or  by  a  magnetic  vane,  which  moves  under 
the  influence  of  the  external  field  magnetism;  in  both 
cases  a  system  of  levers  is  made  to  operate  the  rocker  arm 
to  which  are  attached  the  brushes.  According  as  the 
load  increases  or  decreases,  the  brushes  are  moved 
forward  or  backward.  Such  a  regulator  governed  by  a 
solenoid  can  be  made  very  sensitive,  as  is  illustrated  in 
the  Thomson-Houston  arc  machine.  The  magnetic  vane 
was  used  on  the  Edison  arc  machine,  now  relegated  to 
history.  The  second  method  of  regulation  depends  upon 
a  solenoid  for  its  action,  but  the  function  of  the  magnet 
is  to  press  together  with  varying  force,  according  to  load, 
a  number  of  carbon  plates  which  constitute  a  shunt  to 
the  field  winding.  If  the  number  of  lamps  in  circuit  is 
increased,  and  the  current  begins  to  fall  off,  the  plunger 
of  the  solenoid  relaxes  its  pull  on  the  lever,  which  con- 
trols the  pressure  on  the  plates,  and  the  conductivity  of 
the  contacts  is  thus  impaired :  less  current  passes  through 
the  shunt  and  more  through  the  field  coil,  and  the  field 
becoming  stronger  the  machine's  E.  M.  F.  is  increased 
sufficiently  to  provide  for  the  additional  lamps.  This 
type  of  regulation  is  found  only  on  the  Brush  arc  light 
machine,  and  gives  great  satisfaction.  It  is  the  simplest 
electrical  regulator  in  use.  Since  the  solenoid  is  oper- 
ated by  the  main  current  it  responds  very  promptly  to 
load  variations,  the  carbon  plates  forming  a  high  non- 
inductive  resistance. 


250  TESTING    OF    DYNAMOS    AND    MOTORS. 

Other  less  well  known  makers  use  one  or  the  other 
of  these  principles. 

The  third  and  simplest  mode  of  regulation  requires  the 
use  of  no  mechanism  whatsoever,  and  depends  upon  the 
reaction  between  armature  and  fields.  As  has  beeri 
pointed  out  in  the  preceding  chapter,  all  series  dynamos 
depend  in  a  measure  upon  armature  reaction  for  regula- 
tion, and  in  this  type  of  machine  the  design  is  such  that 
the  regulation  is  close  from  no  load  to  full  load. 

On  account  of  the  demagnetizing  effect  of  the  arma- 
ture, an  arc  machine  can  be  short  circuited  without 
injury,  such  action  resulting  in  removal  of  the  load, 
accompanied  by  some  sparking.  This  method  might  be 
used  for  removing  the  load  ordinarily,  but  short  circuit- 
ing the  fields  alone  accomplishes  the  same  object  and 
without  sparking,  besides  rendering  the  machine  abso- 
lutely inactive.  To  short  circuit  the  armature  brings  its 
reaction  to  a  maximum,  and  this  might  reverse  the  field 
polarity,  thereby  causing  the  arc  lamps  to  burn  upside 
down.  This  armature  reaction  gives  rise  to  peculiar 
behavior  at  times,  and  the  complications  which  it  causes 
in  certain  tests  will  be  considered  later. 

As  over  90^  of  the  world's  arc  lighting  is  done  by 
Thomson-Houston  and  Brush  arc  machines,  a  more  par- 
ticular study  of  these  machines  will  be  in  order. 

Thomson- Houston  Arc  Dynamo. — Professor  Sylvanus  P. 
Thompson  says  of  this  machine:  "  Its  spherical  arma- 
ture is  unique  among  armatures;  its  cup-shaped  field 
magnets  are  unique  among  field  magnets;  its  three- 
part  commutator  is  unique  among  commutators."  The 
armature  is  of  the  open  coil  type,  but  is  wound  in  a 
peculiar  way.  The  inside  ends  of  the  three  coils  are 


THE    SERIES    MACHINE.  25! 

soldered  together,  while  the  outside  ends  connect  each 
to  one  commutator  bar.  In  the  earlier  machines 
the  armatures  were  hand  wound  as  follows:  first,  half 
of  the  first  coil  was  put  on;  then  half  of  the  second 
coil;  then  the  whole  of  the  third  coil,  next,  the  second 
half  of  the  second  coil,  and  finally  the  first  coil  is 
finished.  By  this  means  the  average  distance  of  each 
coil  from  the  core  is  made  the  same.  In  the  more  recent 


F 


FIG.  92. 

types  the  coils  are  lathe  wound  and  are  laid  onto  the 
core,  which  has  a  removable  section.  This  cheapens  the 
cost  of  production,  and  greatly  facilitates  their  repair. 
The  operation  and  regulation  of  the  machine  can  be 
seen  in  Fig.  92.  The  three  coils  of  the  armature  are 
indicated  by  the  figures,  i,  2,  3.  The  current  leaves  the 
armature,  A,  by  the  brushes  B\  B^  passes  through  the 
field  F^  through  the  regulating  apparatus  and  out  on 
the  line,  returning  to  the  machine  through  field,  F^  and 
brushes  B\  Bv  The  regulation  of  the  machine  is  effected 
by  means  of  the  magnets  M  and  r,  while  5  is  a  high 
resistance  carbon  shunt.  The  magnet  c  is  the  controller 
and  is  adjusted  by  means  of  a  spring,  so  that  the  rated 


252  TESTING    OF    DYNAMOS    AND    MOTORS. 

lamp  current  will  just  raise  the  cores,  and  separate  K, 
If  A"  is  closed,  the  current  in  J/,  which  is  the  regulator 
magnet  operating  the  lever  arm  and  brushes,  becomes 
weaker  and  the  core  in  M  drops  by  gravity.  Let  us  sup- 
pose that  the  current  in  c,  and  hence  in  the  lamps,  is  too 
small:  the  cores  in  c  drop,  K  closes,  the  current  in  M 
becomes  smaller,  the  regulator  core  drops,  and  brushes 
B^  and  B^  are  shifted  backward  while  B\  B\  are  shifted 
forward.  This  drawing  together  of  each  pair  of  brushes 
shortens  the  period  of  short  circuit  of  each  coil,  lengthens 
the  time  that  two  coils  are  in  series,  hence,  raises  the 
E.  M.  F.  and  with  it  the  line  current.  If  <r's  current  gets 
too  large,  the  reverse  action  takes  place;  K  opens,  M 
rises,  and  the  pairs  of  brushes  separate,  the  period  of 
short  circuit  is  increased,  and  the  voltage  lowered. 
The  resistance  6"  takes  up  the  spark  when  A"  is  opened, 
and  a  dash  pot  eases  off  the  motion  of  M.  The  current 
obtained  from  the  machine  is  always  in  the  same  direc- 
tion, but  fluctuates  between  zero  and  its  maximum  value 
no  less  than  thirteen  times  *  per  revolution.  These 
fluctuations  are  too  rapid  to  have  any  perceptible  effect 
on  the  working  of  the  lamps,  but  undoubtedly  produce 
induction  effects  on  neighboring  telephone  circuits. 

All  direct  current  series  dynamos  spark  somewhat  upon 
starting,  because  any  regulating  device  requires  time  to 
take  hold.  Ordinarily  the  brushes  should  not  spark  to 
any  extent  under  load,  or  even  overload.  In  open  coil 
armatures  the  sparking  is  greater  than  in  closed  coil 
ones,  because  in  them  each  coil  is  alternately  cut  out  and 
cut  into  circuit  during  every  revolution.  In  the  T.-H. 

*  See  paper  by  M.  E.  Thompson,  read  before  the  A.  I.  E.  E.,  May 
21,  1891. 


THE    SERIES    MACHINE.  253 

machine  this  effect  is  increased  on  account  of  the  large 
angle  covered  by  each  coil;  the  coil  being  often  cut  out 
of  circuit  while  generating  a  strong  E.  M.  F.  The 
method  by  which  the  sparking  is  controlled,  and  even 
made  use  of,  is  very  ingenious.  In  the  first  place  we 
must  note  that  during  part  of  each  revolution  each  coil 
is  short  circuited  by  each  pair  of  brushes,  /.  e.,  when  two 
commutator  segments  touch  two  brushes  which  are  con- 
nected, the  included  coil  short  circuits  through  the 
brushes.  Since  this  short  circuiting  takes  place  in  a  by 
no  means  weak  field,  the  current  in  the  coil  is  consider- 
able, and  so  also  the  danger  of  breaking  down  the  insula- 
tion, if  the  current  is  too  suddenly  broken.  First  then 
it  is  necessary  to  guard  against  a  too  sudden  breaking  of 
the  current,  and  this  is  done  by  the  spark  which  acts 
precisely  as  the  carbon  shunt  S  in  the  controller.  The 
machine,  however,  is  generating  a  very  high  E.  M.  F., 
and  were  this  spark  allowed  to  persist  unmodified  it 
would  heat  the  air  between  the  commutator  bars,  and 
cause  flashing  from  brush  to  brush.  Furthermore,  in 
order  that  the  commutator  may  run  smoothly  and  last 
long,  it  must  be  oiled,  and  this  adds  to  the  danger  of 
flashing.  The  air  blast  was  introduced  to  suppress  the 
spark  after  it  has  served  to  free  the  coil  of  its  current  of 
self-induction,  and  to  introduce  a  thin  layer  of  cold  air 
in  place  of  the  heated  air  due  to  the  spark.  This  air  blast 
makes  it  possible  to  run  a  machine  to  as  high  as  4,500 
or  5,000  volts,  where  without  it  500  volts  would  cause 
flashing.  The  value  of  the  air  blast  is  most  keenly  felt 
when  it  gets  out  of  order.  With  each  machine  a  brush 
gauge  is  sent,  by  means  of  which  the  four  brushes  may  be 
given  the  proper  lead  and  correct  relative  positions. 


254 


TESTING    OF    DYNAMOS    AND    MOTORS. 


The  brushes  constitute  the  "cut-out,"  so  called  because 
they  cut  out  each  coil  a  part  of  every  revolution.  Care 
must  be  taken  that  the  angle  of  the  brush  holder  is  cor- 
rect, and  that  the  distance  of  each  brush  holder  is  exactly 
the  same,  as  measured  on  the  straight  edge  of  the  gauge. 
General  directions  for  brush  setting  are  as  follows — par- 


FIG.  93. 

ticular  directions  accompany  each  machine:  Set  the 
brushes  in  an  exactly  straight  position;  turn  the  com- 
mutator in  the  direction  of  rotation  until  there  is  just 
contact  between  brush  J?4  and  the  end  of  the  segment^', 
(see  Fig.  93).  In  this  position  the  tip  of  brush  J3l  should 
hang  over  the  edge,  C,  of  the  segment  A"  one  sixty- 
fourth  (1/64)  of  an  inch.  Now  turn  the  commutator  till 
the  brush  B^  just  makes  contact  with  the  point  C  on 
segment  A" ';  the  brush  B^  should  hang  over  the  follow- 
ing segment  1/64  inch.  Having  set  the  brushes  properly, 
next  see  that  the  air  blast  is  adjusted;  to  do  this,  the 
regulator  arm  must  be  raised  and  fixed  at  its  extreme 
height;  then  the  jet  is  put  in,  and  the  point  brought  to 


THE    SERIES    MACHINE.  255 

within  about  1/32  inch  in  front  of  the  brush,  and  the 
same  distance  above  the  face  of  the  commutator.  In 
small  machines  the  air  blast  is  sometimes  omitted,  and 
when  tliis  is  the  case  the  spark  should  be  about  1/16" 
for  full  load.  With  the  air  blast  the  spark  is  increased 
to  1/8"  and  on  large  machines  should  be  3/16".  The 
proper  working  of  a  T.-H.  arc  machine  depends  upon 
the  lead  of  the  commutator,  and  upon  the  proper  adjust- 
ment of  air  blast  and  "cut-out"  securing  the  correct 
length  of  spark.  Often,  important  conclusions  can  be 
drawn  from  the  character  of  the  spark,  so  that  the  fol- 
lowing points  should  be  carefully  noted  : 

Possible  causes  of  excessive  flashing  at  the  commuta- 
tor: (i)  The  air  blast  may  be  loose  on  the  back  plate; 
(2)  Air  screens  may  be  stopped  with  dirt;  (3)  Air  blast 
"wings  "  may  be  in  end  for  end — top  edge  should  be  in 
front;  (4)  Air  blast  jet  may  be  jammed;  (5)  Air  blast 
jet  may  be  set  too  far  from  the  ends  of  the  brushes;  (6) 
Air  blast  pins  may  have  been  changed  in  position  and 
be  incorrectly  adjusted;  (7)  Air  blast  jet  may  not  be 
properly  set  with  respect  to  the  commutator  slots;  (8) 
The  commutator  may  be  set  wrong;  the  mark  on  the 
commutator  and  that  on  the  machine  should  line  up; 
(9)  The  "cut-out"  may  be  too  strong;  (10)  The  regula- 
tor yoke  connections  may  be  sticky;  (n)  There  may  be 
a  poor  contact  in  the  controller;  (12)  The  dash  pot  may 
be  either  too  stiff  or  too  weak;  (13)  There  may  be  too 
much  oil  used  about  the  blower,  or  the  oil  may  be  of 
inferior  quality;  (14)  Too  frequent  wiping  of  commuta- 
tor may  have  accumulated  lint,  which,  by  carbonizing, 
short  circuits  the  commutator;  (15)  The  machine  may 
be  overloaded,  or  the  belt  may  be  slipping;  (16)  There 


256  TESTING    OF    DYNAMOS    AND    MOTORS. 

may  be  either  a  short  or  open  circuit  in  the  armature  or 
field;  (17)  There  may  be  a  portion  of  the  circuit  which 
is  periodically  cut  out  by  contact  to  ground,  as  by  wires 
swinging  in  the  wind;  (18)  There  may  be  bad  or  dirty 
controller  contacts  which  keep  the  regulator  cut  in  almost 
constantly.  Many  of  the  above  cases,  especially  those 
due  to  the  presence  of  dirt,  should  never  occur,  and 
some  of  the  remaining  ones  can  be  quickly  eliminated. 
A  contact  or  ground  in  a  single  coil  of  the  armature 
makes  a  sputtering,  broken  sparking  at  the  commutator, 
the  current  surges,  rising  high,  then  falling  low.  This 
surging  may  also  be  caused  by  a  lack  of  sensitiveness  in 
the  controller  magnet,  for  this  makes  the  regulator  rise 
above  then  fall  below  the  normal  points  of  variation. 

The  machine  will  generate  a  heavy  current  if  by 
grounds  or  other  short  circuit  part  of  the  turns  on  the 
regulator  magnet  are  cut  out;  this  weakens  the  magnet 
and  allows  the  regulator  to  fall  too  low.  On  the  other 
hand  a  short  circuit  in  the  field  coil,  cutting  out  part  of 
the  turns,  weakens  the  field's  intensity,  and  lowers  the 
capacity  of  the  machine.  Two  grounds,  one  on  each 
end  of  the  field  coil,  deprives  the  machine  of  generating 
power.  If  the  machine  fails  to  carry  the  standard  num- 
ber of  lights,  the  failure  may  be  due  to  a  short  circuited 
armature.  To  test  for  this,  turn  off  the  lights  and  run 
for  a  short  time  on  dead  short  circuit,  watching  the  am- 
meter to  see  that  the  current  is  not 'too  high;  then  if 
there  is  no  short  circuit  the  regulator  armature  should 
be  up  to  within  r/i6"  of  the  bottom  of  the  regulator.  If 
there  is  a  short  circuit  the  regulator  armature  will  hang 
down  from  1/4  to  1/2  inch.  A  short  circuit  in  any 
armature  can  be  detected  by  running  it  in  separately 


HIE    SERIES    MACHINE.  257 

excited  fields,  and  on  open  circuit;  upon  holding  a  piece 
of  iron  in  the  air  gap  near  one  pole  a  vibrating  puil  can  be 
felt  if  the  armature  has  a  cross  in  it.  The  vibration  is 
unmistakeable,  and  is  due  to  the  fact  that  the  faulty 
armature  is  a  stronger  magnet  when  the  cross  is  in  one 
part  of  the  field  than  in  another,  thus  giving  rise  to  an 
alternately  stronger  and  less  strong  attraction  for  the 
bit  of  iron.  If  there  is  a  cross  the  armature  will,  if  run 
long  enough,  grow  hot  and  ultimately  burn  out  the  fault. 
In  recent  types  of  armature  the  short  circuited  section 
alone  will  heat  and  may  be  thus  located,  but  in  the  old 
types  this  is  not  possible. 

Short  circuit  in  the  field  can  be  roughly  guessed  at  by 
noting  which  pole  pulls  strongest  on  a  piece  of  iron;  or 
better,  the  resistance  of  each  field  can  be  measured 
directly  or  calculated  by  use  of  Ohm's  law. 

A  ground  between  armature  and  shaft  can  be  detected 
by  connecting  each  brush  alternately  to  the  frame  by 
means  of  a  "jumper"  (/'.  e.,  short  wire).  Flashing  at 
the  commutator  results  if  there  is  a  ground.  The  use  of 
the  jumper  will  also  cause  flashing  if  there  is  a  ground 
between  regulator  and  frame.  These  grounds  may  be 
too  undeveloped  to  show  up  through  a  magneto,  and 
should  be  subjected,  as  above,  to  the  full  voltage  of  the 
machine.  At  all  events  they  should  be  located  and 
removed,  as  it  is  hazardous  to  run  so  high  a  potential  on 
a  grounded  circuit  supposed  to  be  insulated.  Should  a 
machine  that  has  been  in  good  working  order  begin  to 
flash,  first  look  over  the*  sliding  connections  on  the 
yokes;  on  old  machines  these  become  weakened  and 
dirty;  next  clean  out  the  blower  and  air  screens,  and  see 
that  the  blower  is  properly  set;  look  over  the  controller 


258  TESTING    OF    DYNAMOS    AND    MOTORS. 

contacts.  If  the  machine  flashes  when  being  loaded 
and  the  spark  becomes  very  short,  and  the  current  runs 
low,  there  is  a  poor,  /.  <?.,  high  resistance,  connection  on 
the  right  lead  to  the  controller,  whereby  the  regulator  is 
not  short  circuited  when  the  controller  contacts  touch. 
If  the  regulator  drops  clear  down,  either  the  regulator 
has  a  short  circuit  or  the  controller  contacts  touch  per- 
manently. 

A  very  long  spark  indicates  that  the  commutator  has 
been  wrongly  set,  and  has  a  long  positive  lead;  the 
regulator  also  drops  to  the  bottom  in  such  a  case.  On 
the  other  hand  if  the  commutator  works  loose  and  slips 
back,  making  a  negative  lead,  the  regulator  arm  is  brought 
up  and  flashing  ensues.  The  lead  ot  the  armature  is  very 
important.  If  the  fields  are  very  strong  a  long  positive 
lead  must  be  given,  otherwise  the  regulator  will  drop, 
the  current  will  be  low,  and  the  spark  long  and  heavy. 
In  rare  cases  a  rheostat  is  placed  in  shunt  across  the 
fields  to  weaken  them  if  it  becomes  necessary.  In  such 
a  case  care  mus.t  be  taken  that  the  box  is  placed  across 
the  fields  and  not  across  the  armature.  To  make  mis- 
takes less  likely,  the  field  connections  must  be  so 
changed  as  to  place  both  spools  on  the  same  side  of  the 
armature,  otherwise  the  shunt  box  must  be  in  two  parts 
to  avoid  shunting  the  armature.  The  cut-out  is  of  equal 
importance  with  the  lead,  and  should  be  so  set  that  the 
current  will  not  be  more  than  4$  or  5$  over  the  rated 
value.  If  the  cut-out  is  set  very  strong,  and  the  lead  at 
the  same  time  be  very  weak  or  negative,  the  regulator 
will  come  up  hard,  and  the  current  will  be  high;  as  the 
load  goes  on,  flashing  is  likely  to  ensue.  In  general, 
where  flashing  is  due  to  wrong  cut-out  and  lead,  as 


THE    SERIES    MACHINE. 


259 


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nj 
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J          I 


260  TESTING    OF    DYNAMOS    AND    MOTORS. 

indicated  by  surging  and  easy  flashing,  the  cut  out 
should  be  weakened  and  the  lead  lengthened.  It  is 
interesting  to  note  that  by  setting  the  commutator  back 
90°,  the  T.-H.  machine  can  be  run  as  a  series  motor, 
though  at  a  very  low  efficiency. 

Enough  has  been  said  to  show  that  a  general  experi- 
ence with  other  machines  does  not  make  one  thoroughly 
at  home  with  a  T.-H.  arc  machine;  and  that  an  intimate 
knowledge  of  the  latter  will  greatly  facilitate  caring  for 
it.  When  understood,  and  intelligently  handled,  the 
machine  easily  holds,  on  its  own  merits,  its  recognized 
place  among  arc  dynamos. 

The  Brush  Arc  Dynamo  is  the  pioneer  of  arc  machines. 
The  first  one  had  its  bobbins  wound  on  a  solid  armature 
core,  and  heated  to  a  wasteful  degree.  This  fault  was 
soon  eliminated.  In  structure  this  excellent  machine 
differs  very  materially  from  other  types.  In  the  first 
place  the  field  poles  confront  either  side  of  the  arma- 
ture, and  hence  the  magnetic  lines  pass  through  the 
armature  parallel,  not  perpendicular,  to  the  shaft.  The 
armature  core  is  therefore  laminated,  not  with  discs,  but 
with  concentric  sheets  formed  by  winding  up  band  iron. 
The  space  between  coils  is  occupied  by  projections 
built  up  of  sheet  iron,  and  serving  the  double  purpose  of 
holding  the  coils  in  position  and  improving  the  magnetic 
circuit.  By  means  of  Figs.  94  and  95  we  see  that  the 
field  spools  are  in  series  with  each  other  and  the  arma- 
ture. To  facilitate  assembling,  the  brass  tips  of  each 
magnet  are  lettered,  those  of  like  letters  connecting 
together.  A  unique  feature  of  the  field  structure  is  that 
between  each  coil  and  the  iron  core  is  a  sheet  of  copper 
soldered  to  form  a  sheath,  which,  by  itself  becoming  the 


THE    SERIES    MACHINE. 


262 


TESTING    OF    DYNAMOS    AND    MOTORS. 


seat  of  induced  currents,  due  to  the  fluctuating  field  cur- 
rent, protects  the  iron  core  from  these  effects. 

The  second  .point  in  which  the  Brush  machine  differs 
from  others,  follows  from  the  first.  Since  the  field  poles 
face  the  armature's  side,  here  are  to  be  found  the  active 
wires,  and  not  on  the  outer  face.  The  armature  is,  there- 
fore, made  as  short  as  is  practicable,  and  somewhat 


A3 


DIAGRAM  OF 
ARMATURE  CONNECTIONS 

•    FOR 
NO.  1 0  &  1 1  ARC  DYNAMO 

FIG.  96. 


resembles  a  large  disc.  Machines  as  now  made  have 
eight,  sixteen,  or  twenty-four  armature  bobbins,  divided 
into  sets  of  four  coils.  The  coils  of  each  set  are  in 
series,  their  final  leads  are  connected  to  adjacent  com- 
mutator bars,  and  alternate  in  position  and  action  with 
those  of  the  other  sets.  The  connections  for  two  sets 
of  bobbins  are  shown  in  Fig.  96.  Each  set  is  indepen- 
dent of  the  others,  is  effective  only  when  its  bars  pass 
under  the  brushes,  so  that  the  machine  is  virtually  two 
machines  with  open  coil  armatures.  The  armature  coils 
are  set  symmetrically  not  only  with  respect  to  each  other 


THE    SERIES    MACHINE.  263 

but  to  the  magnetic  field.  All  four  bobbins  are  in  the 
maximum  field  together,  and  in  zero  field  together. 
The  E.  M.  F.  in  each  set  therefore  rises  to  a  maximum, 
then  falls  to  zero.  Fig.  97  shows  the  character  of  the 
E.  M.  F.  and  current  curves  of  an  eight-coil  machine. 
Curve  I  is  due  to  one  set  of  coils,  and  II  to  the  other 
set  45°  behind  the  first.  The  heavy  line  denotes  the 
resulting  E.  M.  F.  at  the  brushes.  In  the  larger 


II 
FIG.  97. 

machines  additional  sets  of  coils  are  used,  making  the 
E.  M.  F.  steadier,  and  giving  a  larger  current  capacity 
or  a  higher  E.  M.  F.,  as  the  design  may  require.  In  the 
twenty-four  coil  machine  shown  in  Fig.  96  we  have  prac- 
tically three  dynamos  of  eight  coils  each,  in  parallel; 
this  is  shown  by  the  three  commutators  which  are  set 
each  1 1  %°  behind  each  other,  corresponding  to  the  rela- 
tive position  of  their  respective  armature  coils.  In  the 
figure  only  eight  of  the  twenty-four  coils  are  shown. 
A^  Alt  A^  A%  are  in  series  and  their  leads  connect  to  bars 
A^  and  A%.  In  Figs.  94  and  95  it  can  be  seen  that  the 
brushes  are  set  90°  apart,  instead  of  180°,  as  on  many 
machines,  indicating  that  the  commutator  is  cross  con- 
nected, /.  e, ,  opposite  bars  are  metallically  connected, 
thus  obviating  the  necessity  of  using  over  one  pair  of 
brushes  per  commutator. 


264  TESTING    OF    DYNAMOS    AND    MOTORS. 

This  compound  commutator  is  the  third  distinguishing 
characteristic  of  the  machine;  it  is  air  insulated,  and  built 
upon  wooden  blocks.  The  bars  are  long  enough  to  keep 
each  set  of  coils  under  the  brush  during  the  period  of 
strongest  action.  The  brush  is  broad  enough  to  cover 
two  bars  side  by  side.  These  bars  overlap  so  that  as  one 
set  of  coils  dies  down  and  is  cut  out,  the  other  set  is 
brought  into  service.  Similar  action  takes  place  in  the 
other  commutators,  so  that  while  the  current  in  each  set 
of  coils  is  periodically  broken,  the  line  current  is  not,  and 
its  fluctuations  are  confined  to  narrow  limits,  though  it 
is  by  no  means  as  steady  as  in  the  ordinary  closed  qoil 
armature  of  many  section's. 

We  next  turn  our  attention  to  the  automatic  regulator 
shown  in  Fig.  98.  The  principle  of  the  regulator  has 
already  been  given,  viz.:  that  of  placing  a  variable  shunt 
across  the  fields,  thereby  controlling  their  magnetization. 
The  duty  of  the  regulator  is  twofold:  First,  it  must  keep 
the  current  constant  by  means  of  the  shunt;  second,  it 
must  keep  the  brushes  in  such  a  position  as  to  give  a 
spark  of  proper  length  to  take  up  the  self-induction  of 
each  coil  as  it  is  cut  out  of  circuit,  and  must  not  let  the 
spark  get  long  enough  to  cause  flashing.  The  regulator 
connections  are  shown  in  Fig.  100.  The  box  bar  sweeps 
across  a  set  of  contacts,  throwing  more  or  less  resistance 
in  shunt  with  the  field  magnet  circuit,  and  at  the  same 
time  rocking  the  brushes  so  that  the  spark  is  kept  con- 
stant; a  small  belt  driven  by  the  armature  shaft  runs  the 
worm  Pt  which  in  turn  operates  the  worm  wheel  Q. 
Attached  to  Q  is  shaft  ^?,  on  which  are  mounted  clutches 
S  and  S'.  7"and  T'  are  the  armatures  for  the  respective 
clutches,  and  to  these  are  rigidly  connected  two  small 


THE    SERIES    MACHINE. 


265 


I       5 


266  TESTING    OF    DYNAMOS    AND    MOTORS. 

bevel  gears,  X  and  X',  both  of  which  engage  the  small 
bevel  gear  W.  W\s  fast  to  shaft  K,  on  which  is  mounted 
the  spur  pinion  Z,  engaging  sector  JV.  The  contact  arm, 
or  box  bar  A,  is  also  attached  to  shaft  Y.  When  the  cur- 
rent increases  above  its  normal  value,  a  certain  amount 
passes  through  clutch  S,  which  immediately  seizes  its 
armature  X,  and  the  contact  arm  sweeps  over  the  con- 
tacts, cutting  out  resistance  in  the  shunt,  weakening  the 

field,  and  restoring  the  current 
to  its  normal  value.  At  the 
same  time  the  brushes  are 
shifted  backward.  In  case 
the  current  falls  below  its 
proper  value,  clutch  S'  acts, 
the  shunting  power  is  decreased,  the  field  thereby  strength- 
ened, and  the  brushes  moved  forward  to  meet  the  condi- 
tion. This  regulator  is  then  a  mechanical  device  kept  in 
continuous  motion  and  electrically  controlled  by  means  of 
what  is  called  the  "wall  controller." 

The  controller,  Fig.  98,  consists  of  two  relays  (electro- 
magnets A' and  Y)  and  a  small  German  silver  resistance. 
The  main  current  enters  the  controller  at  the  right-hand 
top  binding  post,  passes  around  the  two  relays  and 
through  the  German  silver  resistance,  and  so  on  out  at 
the  upper  left-hand  binding  post.  At  the  point  A  the 
circuit  is  connected  to  the  yokes  of  the  relays,  and  in 
metallic  connection  with  this  are  two  silver  contacts,  B 
and  C\  opposite  these  are  two  platinum  contacts,  B'  and 
C ',  which  connect  to  posts  JF  and  E.  In  case  the  current 
rises  above  its  normal  value,  relay  X  raises  its  armature 
and  makes  B  touch  B'.  If  the  current  falls  too  low, 
relay  K  drops  its  armature,  letting  C  touch  C',  and  cur- 


THE    SERIES    MACHINE. 


267 


rent  passes  down    through  E.     In  order  to   reduce  the 
sparking  at  the  make  and  break  contacts,  a  small  spool  of 


FIG.  100. 

German  silver  wire  is  connected  across  the  breaking 
point,  to  take  up  the  secondary  current  due  to  the  self- 
induction  in  the  clutch.  The  principle  of  its  action  can 


268  TESTING    OF    DYNAMOS    AND    MOTORS. 

be  seen  by  aid  of  Fig.  99,  which  shows  the  connection 
on  the  field  of  a  street  railway  generator.  We  have  seen 
that  when  we  break  a  circuit  containing  an  electro- 
magnet, the  induced  E.  M.  F.  is  high  and  causes  a  long 
spark,  besides  straining  the  insulation.  In  Fig.  99  F  is 
the  generator  field,  L  some  lamps  whose  number  had  best 
be  determined  experimentally,  K  is  a  switch,  and,  when 
the  dynamos  are  working,  occupies  position  i.  If  it  is 
desired  to  break  the  generator  field,  K  is  thrown  over  to 
position  2,  which  includes  the  lamps,  whose  resistance  is 
so  high  that  the  machine  cannot  support  its  field  through 
them.  In  the  regulator  case  before  us,  the  lamps  are  re- 
placed by  the  high  resistance  German  silver  wire,  which 
instead  of  being  placed  in  with  a  switch  is  in  all  the  time, 
and  when  B B1  or  CC'  opens,  the  induced  current  is 
given  the  alternative  of  an  air  gap  or  this  German  silver 
resistance.  The  method  of  connecting  the  wall  control- 
ler and  automatic  regulator  to  the  machine  is  shown  in 
Fig.  94.  Facing  the  commutator  the  left-hand  binding 
post,  as  indicated,  must  always  be  positive,  and  the  wire 
from  this  post  should  run  to  the  line  direct.  On  return- 
ing to  the  station  the  wire  passes  through  the  ammeter 
and  thence  to  the  right-hand  post  A  '  of  the  wall  con- 
troller, through  the  controller  out  at  B\  and  to  the  right- 
hand  upper  post  B  on  the  dynamo;  from  here  it  passes 
through  the  field  and  back  to  the  lower  post  on  the  same 
side  of  the  dynamo,  and  from  here  on  through  the  arma- 
ture. The  lower  binding  posts,  F'  and  £',  on  the  wall 
controller,  connect  respectively  to  F  and  E  on  the 
clutches  S  and  S'.  From  the  clutches  the  current  passes 
to  the  centre  ring  of  the  governor.  In  Fig.  100  we  see 
that  this  ring  connects  to  the  small  binding  post./)', 


THE    SERIES    MACHINE.  269 

which  in  turn  connects  to  the  small  upper  binding  post 
D  on  the  right-hand  side  of  the  machine.  The  connec- 
tion between  binding  post  E  on  S'  is  of  use  only  when 
the  machine  is  overloaded.  For  example,  suppose  the 
current  to  fall  below  9.6  amperes,  the  right-hand  relay  of 
the  wall  controller  acts  and  sends  current  through  the 
right-hand  clutch  S,1  which  tends  to  rotate  its  own  arma- 
ture; all  resistance  in  the  shunt  rheostat  is  cut  in  and 
the  brushes  are  back  as  far  as  they  can  go.  To  relieve 
clutch  S'of  its  current,  thereby  taking  off  P,  a  strain 
which  would  soon  wear  it  out,  the  clutch  S',  in  this 
extreme  position  of  the  contact  arm  A,  is  short  circuited. 
In  all  other  positions  of  A  the  resistance  box  is  in  shunt 
with  the  field  through  binding  post  C',  which  connects 
to  lower  post  Con  the  machine,  while  D'  connects  to  D, 
thereby  throwing  the  resistance  and  field  in  shunt. 

Before  starting  up  a  machine  see  that  its  rocker  arm 
moves  freely;  a  little  thin  oil  can  be  used  here  to  advan- 
tage. Care  should  be  taken  to  have  no  cramping  where 
the  teeth  of  the  small  spur  pinion  Z  engage  the  teeth  of 
the  sector  JV  (see  Fig.  100).  A  small  amount  of  oil 
should  be  placed  in  the  oil  chamber  surrounding  the 
worm  to  insure  its  lubrication.  It  is  well  to  test  the  re- 
lays in  the  wall  controller  before  throwing  on  the  load. 
To  do  this,  rock  the  brushes  well  forward  and  see  that 
the  contact  arm  A  is  on  the  extreme  right.  Connect  the 
main  binding  post  A  (Fig.  94)  of  the  dynamo  to  post  A^ 
of  the  ammeter;  disconnect  E'  and  F'  on  the  wall  con- 
troller, then  start  the  machine  and  adjust  the  right-hand 
relay  so  that  it  will  drop  its  armature  when  the  current 
falls  below  9.4  amperes,  and  the  left-hand  relay  so  it 
will  raise  its  armature  when  the  current  exceeds  9.8 


270  TESTING    OF    DYNAMOS    AND    MOTORS. 

amperes.  Reconnect  E  and  F,  close  the  field  armature 
switch,  rock  the  brushes  back  and  the  machine  is  ready 
for  load.  The  spark  must  then  be  adjusted  to  proper 
length  by  loosening  clamp  L  (Fig.  100),  and  rocking  the 
brushes  backward  or  forward  as  may  be  necessary;  then 
reset  the  clamp  and  the  regulation  will  be  automatic  for 
all  load  variations.  It  may  be  necessary  to  make  a  final 
adjustment  of  the  controller  relays  after  the  load  is 
actually  on  the  machine. 

In  this  machine  as  in  all  others,  failure  to  operate 
satisfactorily  is  frequently  due  not  to  any  fault  in  the 
machine  itself,  but  to  inefficient  knowledge  of  details 
as  to  connections,  switches,  commutators,  regulators, 
brushes,  and  bearings.  Some  practical  points  will,  then, 
be  in  place.  To  aid  in  distinguishing  between  the  inside 
and  outside  field  spool  terminals,  the  inside  ones  are 
painted  red,  and  are  also  tagged  before  leaving  the  shop. 
The  field  switch  should  be  mounted  on  the  same  side  of 
the  machine  as  the  rocker  arm.  In  handling  the  brushes, 
always  close  the  small  switch  located  just  above  the  com- 
mutator, short  circuiting  the  armature,  otherwise  the 
attendant  is  liable  to  get  a  severe  shock,  even  though 
the  field  switch  be  closed.  To  "kill"  the  machine  in- 
stantaneously, as  in  case  of  accident,  close  the  armature 
switch.  Under  ordinary  circumstances  it  is  best  to  close 
the  field  switch  first.  In  starting  up  it  is  immaterial 
which  is  opened  first. 

To  set  the  brushes  a  sheet  metal  gauge  is  used.  This 
gauge  is  shipped  with  the  machine.  Sometimes  after  the 
brushes  are  uniformly  adjusted  it  will  be  found  that  the 
spark  varies  on  the  different  commutators.  This  is 
owing  to  lack  of  uniformity  in  cross-section  of  different 


THE    SERIES    MACHINE.  271 

parts  of  the  armature  and  to  local  variations  in  its 
magnetic  quality.  This  is  unavoidable,  and  in  such  cases 
the  brush  should  be  lengthened  or  shortened  from  1/16" 
to  1/32"  to  suit  the  circumstances.  Under  ordinary  cir- 
cumstances i/8"  is  the  proper  length  of  spark,  but  where 
large  fluctuations  of  load  are  likely  to  occur,  as  for 
example  where  hotels,  concert  halls,  skating  rinks,  etc., 
are  in  circuit,  it  is  well  to  carry  a  3/16"  or  1/4"  spark, 
because  where  variations  are  wide  and  sudden  a  great 
many  lines  of  force  are  put  in  motion  and  the  self-induc- 
tion is  heavy.  All  series  machines,  and  especially  those 
which  depend  to  any  degree  on  armature  reaction  for 
regulation,  give  lower  and  lower  efficiencies  as  the  load  is 
decreased.  This  is  so  on  the  Brush  machine,  because  the 
fewer  the  lamps  in  circuit  the  less  is  the  voltage  needed, 
and  hence  the  more  the  current  which  must  pass  through 
the  shunt  and  be  wasted.  On  the  Thomson-Houston 
machine  running  light,  the  coils  are  short  circuited  for  so 
long  a  time  during  each  revolution,  that  they  may  be  in- 
active, as  far  as  concerns  the  external  load,  as  long  or 
longer  than  they  are  active.  The  local  current  in  the 
short  circuited  coils  gives  rise  to  heat,  and  impose 
additional  drag  on  the  engine  without  making  any  return 
for  it.  On  either  of  these  arc  machines  the  iron  losses 
are  greater  under  light  load,  because  the  fluctuation 
and  hence  movement  of  lines  of  force  is  greater.  If  the 
machine  supplies,  say  i  lamp,  whose  resistance  equals 
that  of  the  machine,  and  if  we  suppose  the  resistance  of 
the  lamp  when  burning  to  be  5  ohms,  the  resistance  of  the 
line  will  be  10  ohms;  and  if  for  any  reason  the  i  lamp  cuts 
itself  out,  the  line  resistance  suffers  a  variation  of  50^, 
whereas  if  there  are  9  lamps  in  circuit,  i  lamp  is  but  i/io 


272  TESTING    OF    DYNAMOS    AND    MOTORS. 

of  the  line  resistance,  and  its  being  cut  out  makes  a  dif- 
ference of  but  10$.  On  those  machines  whose  regulation 
depends  on  armature  reaction  (such  as  the  Edison  and 
Westinghouse  direct  current  arc  machines),  it  is  inefficient 
to  run  on  light  loads,  because  then  the  armature  reaction 
is  greatest,  and  the  field  lines  of  force  are  created  only 
to  be  neutralized  by  those  of  the  armature,  whereas 
at  full  load  the  armature  has  little  positive  influence, 
and  all  the  lines  of  force  generated  are  used.  It  is 
safe  to  say,  then,  that  no  arc  machine  should  run  on 
light  load  any  longer  than  can  be  helped.  To  revert  to 
the  Brush  machine  in  particular: — its  commutator  should 
have  the  best  care,  for  upon  it  satisfactory  working 
largely  depends.  It  should  be  kept  perfectly  clean  and 
only  enough  oil  used  to  prevent  cutting;  this  should  be 
mineral  oil,  and  should  be  applied  with  a  piece  of  felt  or 
leather.  To  insure  safety  to  the  attendant  it  is  abso- 
lutely necessary  that  the  felt  or  leather  be  applied  from 
a  dry  stick,  piece  of  fibre,  or  other  insulator  at  least  twelve 
inches  long.  Machines  whose  E.  M.  F.  range  from  3,000 
to  6,000  volts  are  dangerous  traps,  unless  respected,  and 
any  mistake  is  apt  to  be  a  fatal  one.  The  commutator 
bars  are  built  upon  hard  wood  blocks  (generally  lignum 
vitse),  and  it  is  sometimes  necessary  to  renew  them;  as 
even  well  seasoned  wood  is  apt  to  be  a  little  damp,  new 
blocks  may  occasion  more  or  less  trouble  at  first.  To 
prevent  flashing,  the  spark  can  be  lengthened  slightly  for 
a  few  hours,  till  the  wood  is  dried  out. 

The  troubles  to  which  the  machine  is  liable  are  in 
general  the  same  as  those  already  spoken  of,  and  the 
same  general  precautions  and  methods  of  testing  are  to 
be  followed.  The  first  sign  of  a  defective  armature 


THE    SERIES    MACHINE.  273 

bobbin  is  seen  on  the  commutator;  the  spark  will  grow 
short  and  may  disappear  at  one  of  the  commutator  sec- 
tions, and  there  may  be  some  flashing.  If  the  machine  is 
badly  needed,  the  brush  can  be  cut  a  little  back  and  the 
machine  may  run  a  long  time  if  not  overloaded.  Upon 
shutting  down,  the  short  circuited  or  defective  coil  can 
be  located  by  the  fact  that  it  will  be  overheated.  As 
there  is  considerable  space  between  bobbins,  an  accumu- 
lation of  dust  sometimes  starts  a  ground  between  bobbin 
and  core.  To  prevent  this  a  stiff  brush  and  bellows 
should  be  used  every  day  or  so.  Dust  and  oil  must  not 
be  allowed  to  gather  on  the  armature  leads  where  they 
enter  the  shaft  and  where  they  come  out  on  the  commu- 
tator end*  oil  is  doubly  harmful  because  it  not  only  holds 
the  dust,  but  rots  the  insulation.  As  a  rule  the  commu- 
tator segments  wear  out  first,  and  at  the  tips,  where  the 
arc  holds.  Their  life  can  be  prolonged  by  taking  them  off 
and  bending  up  the  tips.  The  tip  can  then  be  filed  down 
to  a  circle  and  replaced ;  next  cut  out  a  block  of  wood  to 
fit  the  commutator;  cover  the  inside  of  the  block  with 
sand-paper  (not  emery),  and  press  it  against  the  com- 
mutator while  the  armature  turns  at  about  half  speed. 
In  order  to  line  all  segments  up  to  the  circumference  of 
the  same  circle  it  may  be  necessary  to  slightly  raise  a 
segment;  to  do  this  slack  up  the  screws  and  pack  under 
the  segment  with  sheet  fibre  or  other  insulator,  and  then 
proceed  with  the  sand-papering.  During  this  operation 
the  leads  should  be  carefully  wrapped  to  prevent  copper- 
dust  from  entering  the  shaft  hole.  The  commutator 
should  be  then  carefully  wiped  off.  The  Brush  regulating 
apparatus  is  perhaps  not  so  compact  as  that  of  the  Thom- 
son-Houston regulator,  nor  is  it  as  purely  an  electrical 


274  TESTING    OF    DYNAMOS    AND    MOTORS. 

device;  nevertheless,  with  due  attention  it  gives  excellent 
satisfaction.  The  dial,  if  kept  in  good  order,  will  for  the 
little  extra  attention,  make  ample  returns,  but  if  neglected, 
will  require  the  assistance  of  the  attendant  to  regulate  the 
lamps,  which  will  either  flame  or  hiss.  It  is  a  good  idea 
to  mount  all  the  station  dials  on  a  piece  of  wood  about 
i  1/2  X  3  inches  square,  and  long  enough  to  run  the  whole 
length  of  the  dial.  The  upper  strip  should  be  hinged 
to  the  wall  frame  or  switchboard.  This  facilitates 
removal  of  carbons  for  inspection  and  cleaning.  In 
replacing  it  should  be  seen  to  that  the  carbon  piles  work 
freely  between  the  dividing  slates,  and  that  they  make 
good  contact  across  from  one  pile  to  the  other. 

There  are  in  use  other  arc  machines  and  regulators, 
which  are  gradually  gaining  a  foothold,  and  which  might 
well  be  described  here,  but  they  are  only  adaptations  and 
modifications  of  principles  above  dwelt  upon,  and  any- 
one who  can  handle  and  understand  the  Thomson-Hous- 
ton and  Brush  machines  will  be  at  a  loss  with  no  other. 

We  therefore  turn  to  the  last  type  of  arc  machine  to 
be  here  considered.  This  is  the  new  Westinghouse 
direct  current  generator.  Though  the  machine  differs 
very  radically  in  construction  from  former  types,  it 
belongs  to  the  open-coil  continuous  current  class,  which 
has  special  advantages  for  arc  work.  It  delivers  a  cur- 
rent which  is  pulsating,  but  always  in  the  same  direction, 
/.  <?.,  the  current  is  not  constant  in  value,  varying 
between  narrow  limits,  and  the  brushes  remain  always  of 
the  same  polarity.  It  is  upon  the  fact  that  the  current 
pulsates  that  the  machine's  automatic  regulative  ability 
depends.  The  fields  are  separately  excited  at  no  volts, 
thus  avoiding  the  introduction  of  high  voltage  into  the 


THE    SERIES    MACHINE.  275 

field  circuit,  and  securing  a  field  current  of  constant 
strength.  The  main  current  has  a  steady  mean  value, 
and  the  pulsations  keep  the  mechanisms  of  the  lamps  in 
a  constant  state  of  mild  vibration,  thereby  making  them 
less  liable  to  become  slow  in  action  or  to  stick.  In  gen- 
eral appearance  the  machine  resembles  the  Westinghouse 
alternator:  consisting  of  a  circular  cast  iron  yoke  parted 
horizontally  and  having  inwardly  projecting  poles.  The 
armature  is  of  the  toothed  type,  having  especially  heavily 
insulated  lathe  wound  bobbins.  Between  the  teeth  are 
wedge-shaped  iron  lugs  to  be  driven  in  after  the  bobbins 
are  in  place.  Each  coil  of  the  armature  consists  of  two 
of  these  bobbins,  which  are  put  on  one  at  a  time,  and  then 
taped  together  on  the  ends.  There  are  eight  armature 
coils  and  but  six  field  spools  and  poles;  this  peculiarity 
is  accounted  for  by  the  fact  that  from  the  nature  of  the 
commutator  two  coils  are  always  idle,  leaving  an  active 
coil  to  each  pole.  The  strong  point  claimed  for  the 
machine  is  its  method  of  automatic  regulation  without 
regulating  devices  of  any  kind.  For  regulation  it  depends 
solely  upon  the  armature  reaction  about  which  we  have 
spoken  above;  the  principles  here  involved  can  be  better 
understood  if  we  look  for  a  moment  at  the  reactive 
effect  of  an  alternator  armature.  In  the  field  winding  of 
an  alternator  the  direction  of  the  current  does  not 
change,  but  that  of  the  armature  flows  first  one  way  and 
then  the  other,  alternating  as  each  pole  piece  is  passed. 
It  can  be  seen  then  that  an  armature  conductor  opposite 
a  pole  piece  may  assist  the  field  coil  and  strengthen  its 
field,  or  may  oppose  the  coil  and  exert  a  weakening  effect, 
according  as  the  conductor's  current  concurs  with,  or 
opposes  the  magnetizing  effect  of  the  current  in  the  coil. 


276  TESTING    OF    DYNAMOS    AND    MOTORS. 

Whether  there  is  opposition  or  concurrence  depends 
upon  the  direction  of  the  current  for  a  given  position  of 
the  conductor.  Assume  for  simplicity  that  our  con- 
ductor lays  midway  between  two  pole  pieces,  and  that 
when  the  armature  carries  its  full  load,  the  conductor 
current  reaches  its  greatest  value  in  this  position.  The 
conductor  being  midway  between  two  poles  exerts 
counter  influences  on  the  rear  and  forward  poles,  and  its 
resultant  influence  is  zero.  If  lamps  are  in  series  and 
several  be  turned  off,  the  effect  is  to  momentarily  increase 
the  current,  and  also  the  self-induction  of  the  circuit. 
Now  if  the  self-induction  of  a  circuit  is  increased  the 
current  does  not  reach  its  maximum  value  as  soon  after 
the  E.  M.  F.  reaches  its  maximum  value  as  it  otherwise 
would,  so  that  in  this  particular  case  the  lagging  current 
no  longer  reaches  its  maximum  value  when  the  conductor 
is  midway  between  pole  pieces,  but  does  so  beyond  this 
point,  and  being  then  nearer  the  approaching  pole  is  in  a 
position  to  exert  a  stronger  demagnetizing  influence  on 
this  pole.  The  result  is  to  reduce  the  E.  M.  F.  and 
restore  the  current  to  its  proper  value.  When  the 
machine  is  short  circuited,  the  self-induction  and  result- 
ing lag  become  so  great  that  the  conductor  has  its  maxi- 
mum demagnetizing  ability  when  it  is  immediately  in 
front  of  a  pole  piece.  As  seen  above  a  pulsating  current 
partakes  of  the  nature  of  an  alternating  current  in  that  it 
fluctuates  between  limits,  but  differs  from  the  latter  in 
that  there  is  no  change  of  sign.  The  current  in  each 
coil  starts  from  zero,  goes  to  a  plus  maximum  and 
returns  to  zero,  to  repeat  the  same  cycle.  The  amount 
of  reaction  between  any  armature  conductor  and  a  pole 
piece  depends  upon  the  conductor's  position,  its  current 


THE    SERIES    MACHINE. 


277 


strength,  and  in  part  upon  whether  this  current  is  rising 
or  falling  in  value.  At  full  load  the  current  in  any  con- 
ductor may  be  at  its  least  value  when  the  conductor  is  in 
the  best  position  for  demagnetizing  the  field;  and  is  at 


TO 

LAMPS 


FIG.  loi. 

its  highest  value  when  best  disposed  to  reinforce  the 
field.  On  short  circuit  the  armature  current  reaches  its 
maximum  value  when  the  conductor  is  opposite  a  pole 
piece  of  similar  sign.  In  these  machines,  as  in  alternators, 
the  shape  and  disposition  of  the  pole  pieces  have  been 
experimentally  determined.  The  machine  in  question 
can  be  better  understood  by  looking  at  Fig.  101.  On 
the  machine  itself  the  two  commutators  occupy  posi- 
tions side  by  side  on  the  shaft,  while  in  the  figure  the 
outside  commutator  represents  the  one  next  the  arma- 
ture, the  inside  small  one  the  one  next  the  end  of  the 


278  TESTING    OF    DYNAMOS    AND    MOTORS. 

shaft.  Notice  that  a  straight  line  through  the  centre  of 
the  shaft  and  the  centre  of  a  bar  on  one  commutator, 
passes  through  the  centre  of  the  insulation  on  the  other, 
showing  the  commutators  to  be  on  the  shaft,  one  a  little 
ahead  of  the  other.  Besides  the  mica  insulation,  there 
are  two  air  gaps  between  each  segment,  one  on  each  side 
of  the  mica.  Points  numbered  the  same  are  in  metallic 
communication,  thus  8,  the  inside  end  of  a  bottom  sec- 
tion, goes  to  binding  post  8,  on  the  end  of  the  shaft, 
and  this  connects  to  three  bars  marked  8,  on  the  little 
or  outside'-commutator.  The  field  circuit  is  completed 
through  a  device  operated  by  the  plunger  of  a  solenoid 
in  the  main  circuit.  If  the  outside  circuit  opens,  the- 
plunger  falls  and  opens  the  field. 


CHAPTER  IX. 

TESTING    OF     DYNAMOS     AND     MOTORS,    SHUNT     AND     COM- 
POUND MACHINES. 

THE  shunt  dynamo  differs  from  the  series  dynamo  in 
that  the  field  circuit  is  independent  of  the  external  or 
line  circuit,  and  its  E.  M.  F.  is  highest  when  the  line 
switch  is  open.  .With  the  fields  connected  below  the 
switch,  the  dynamo  generates  on  open  circuit  only  such 
current  as  the  existing  E.  M.  F.  of  the  machine  can  urge 
through  the  combined  resistance  of  the  field  and  its 
rheostat.  The  amount  of  current  which  will  pass  through 
any  resistance  depends  upon  the  potential  difference  at 
its  terminals,  and  conversely,  the  current  which  any 
given  potential  difference  will  send  through  a  circuit  de- 
pends upon  the  resistance  of  the  circuit;  and,  further, 
we  know  that  any  applied  E.  M.  F.  distributes  itself 
around  a  circuit  according  to  the  disposition  of  the  re- 
sistance. Wherever  the  greatest  resistance  is  found  there 
also  is  the  greatest  potential  difference  per  unit  of  length 
of  circuit;  and  when  a  circuit  is  broken,  the  resistance  of 
the  air  gap  at  the  break  is  so  great  compared  with  the 
resistance  anywhere  else  that  the  entire  potential  differ- 
ence takes  place  across  the  break.  On  the  other  hand, 
if  between  any  two  points  a  second  wire  be  placed,  as 
between  points  A  and  B  in  the  circuit  of  Fig.  102,  the 
effect  is  to  decrease  the  resistance  between  these  points, 

279 


280  TESTING    OF    DYNAMOS    AND    MOTORS. 

since  there  are  now  two  paths  instead  of  one,  and  thereby 
to  decrease  their  difference  of  potential.  The  original 
path  A  D  B  being  now  subjected  to  a  less  potential  dif- 
ference, does  not  get  as  much  current  as  it  did  before 
being  shunted.  If,  however/  by  any  means  (increasing 
the  E.  M.  F.  applied  to  the  circuit)  the  potential  differ- 
ence between  A  and  B  be  raised 
to  its  initial  value,  path  A  D  B 
will  take  the  same  current  that  it 
had  when  alone  in  circuit,  and 


D  path  A  c  B.  a  current  depending 

FIG.  102. 

upon  its  resistance.       1  he  total 

current  has  been  increased.  We  may  say,  then,  that 
the  current  taken  by  any  branch  is  independent  of 
that  flowing  in  any  other  branches,  provided  the  poten- 
tial difference  at  the  common  junction  is  maintained 
the  same,  and  the  current  which  each  branch  takes 
depends  upon  its  own  resistance.  If  the  potential  dif- 
erence  between  A  and  B  is  not  regulated,  then  the 
effect  of  introducing  an  additional  path  is  to  divert  the 
current  from  existing  paths,  and  in  a  degree  depending 
upon  its  conductivity.  Both  conditions  can  be  practically 
illustrated  by  means  of  Fig.  103,  where  A  is  the  armature, 
/"the  field,  and  A" and  Z,  respectively,  the  line  switch  and 
lamp  load  of  a  shunt  dynamo.  The  smaller  lettering  is 
identical  with  that  of  Fig.  102.  The  armature  A  applies 
its  E.  M.  F.  at  a  and  b,  the  common  terminals  of  field 
and  line.  When  K  is  open  A  and  F  are  in  series,  and  all 
the  current  A  generates  goes  through  F.  The  amount  of 
this  current  depends  upon  A's  E.  M.  F.  and  upon  jF's  re- 
sistance. A's  E.  M.  F.  depends  upon  its  speed,  its  num- 
ber of  conductors,  and  the  number  of  lines  of  force  they 


SHUNT    AND    COMPOUND    MACHINES.  281 

cut.  The  number  of  lines  of  force  depends  in  turn  upon 
the  field  current.  With  A  and  F  alone  in  circuit,  A's  re- 
sistance being  very  low,  the  total  potential  drop  takes 
place  through  F's  high-resistance  windings,  and  can  be 
measured  between  a  and  b.  Potential  measured  under 
this  condition  is  called  the  "  open  circuit "  potential  of 
the  dynamo.  When  K  is  closed,  resistance  between  a 
and  b  becomes  less,  and  the  armature  current  increases,  so 
that  there  is  a  redistribution  of  potential.  We  know  that 
the  drop  between  a  and  b  is  the  machine's  useful  E.  M.  F., 
and  that  the  drop  through  the  armature  resistance  is  a 
dead  loss,  and  equals  the  product  of  armature  resistance 
and  current,  or  =  7/'a,  where  /is  A's  current  in  amperes, 
and  /•„  its  resistance  in  ohms.  The  final  effect  then  of 
closing  K  is  to  decrease  the  circuit  resistance  and  in- 
crease the  current.  The  increased  current  causes  a 
greater  drop  to  take  place  through  the  armature,  and 
hence  decreases  the  E.  M.  F.  available  at  the  line  termi- 
nals; but  since  the  field  winding  is  energized  by  the 
potential  difference  between  these  points,  when  this 
grows  less,  the  field  gets  less  current,  the  magnetism 
decreases,  and  the  voltage  does  too.  To  determine 
what  part  of  the  voltage  decrease  is  due  to  increased 
armature  drop,  and  what  part  is  due  to  decreasing  the  field 
current,  proceed  as  follows  :  Get  the  dynamo  up  to 
voltage,  throw  on  a  certain  number  of  lamps,  and  observe 
the  loss  of  potential  between  a  and  b.  This  loss  will  be 
due  to  the  combined  effect  of  the  above  two  causes;  next 
separately  excite  the  field,  bring  the  armature  up  to 
voltage,  and  again  throw  on  the  lamps.'  The  loss  is  now 
due  entirely  to  increase  of  armature  drop,  consequent 
upon  increasing  the  current.  To  be  strictly  correct,  the 


282  TESTING    OF    DYNAMOS    AND    MOTORS. 

armature  should  carry  the  same  current  in  both  cases.  In 
Fig.  103  the  armature  is  the  undivided  part  of  the  circuit, 
and  the  branches  F  and  L  the  divided  part.  If  we  call  / 
the  current  in  the  undivided  part,  and  i\  and  i\  that  in  the 
branches,  then/a  -f-  /a  =  /,  and  t\  :  /2  ;  ;  ;-a  :  r^  where  ra  and 
fj  are  the  respective  resistances  of  branches.  If  we  call 

r&  the  armature  resist- 
ance, then,  since  from 
Ohm's  law  the  drop  is  in 
any  case  the  product  of 
current  by  resistance, 
we  have  the  armature's 
"  lost  volts  "  =  7  ra,  and 
in  designing  armatures 
allowance  must  be  made 
for  this  loss,  which  must  be  calculated  when  the  armature 
is  hot,  not  cold,  for  as  more  lamps  are  thrown  across  the 
mains,  the  external  resistance  is  further  decreased,  a 
larger  flow  of  current  takes  place  through  the  armature, 
giving  rise  to  heat,  and  robbing  the  fields  of  more  mag- 
netizing power.  A  shunt  dynamo,  with  an  armature  of 
zero  resistance,  would  be  self-regulating  for  constant 
potential  at  the  brushes,  for  without  resistance  there 
could  be  no  loss  of  potential  in  the  armature,  and  hence 
the  total  E.  M.  F.  would  always  be  the  same  as  that  at 
the  brushes.  This  ideal  state  is  approached  by  mak- 
ing the  armature  resistance  as  low  as  possible.  A  shunt 
dynamo  is  theoretically  adapted  to  automatically  regulate 
for  constant  potential  at  the  brushes,  when  the  lamps  or 
other  devices  are  in  series.  As  the  number  of  lamps 
increases  so  does  the  circuit  resistance,  and  with  it  the 
potential  at  the  field  circuit  terminals,  and  hence  the  ma- 


SHUNT    AND    COMPOUND    MACHINES.  283 

chine's  ability  to  supply  an  increased  voltage;  but  where 
devices  are  in  series  they  all  take  the  same  current,  so 
that  the  line  voltage  must  be  very  high  to  supply  any 
considerable  amount  of  energy.  A  shunt  winding  which, 
when  subjected  to  very  high  voltage,  would  give  the  nec- 
essary magnetizing  power,  and  at  the  same  time  be  of 
sufficiently  high  resistance  to  shunt  an  economical  frac- 
tion of  the  armature  current,  would  be  very  expensive  to 
make.  The  cost  of  insulated  wire  per  pound  increases 
very  rapidly  as  the  diameter  decreases.  A  pound  of  No. 
5,  double  cotton-covered  B.  &  $.,  copper  wire  sells  for 
14  1/2  cents  a  pound,  of  No.  10  at  17  1/2  cents,  No.  20 
at  28  cents,  etc.  As  the  wire  grows  smaller  its  insulation 
takes  up  a  relatively  larger  and  larger  part  of  the  winding 
space,  till  when  we  reach  a  No.  13  B.  &  S.  the  copper  and 
cotton  occupy  about  the  same  space.  This  means  that 
in  order  to  get  on  enough  copper  the  machine  must  be 
made  larger  than  its  output  would  call  for,  and  the  maker 
would  lose  the  interest  on  money  uselessly  expended. 
The  larger  a  wire,  the  smaller  is  the  relative  amount  of 
space  taken  up  by  insulation.  On  a  series  machine 
the  whole  armature  current  flows  through  the  field  wind- 
ing, so  that  fora  given  magnetization  there  need  not  be  as 
many  turns  of  wire.  Accordingly,  under  the  dictates  of 
economy,  series  machines  have  been  adopted  in  high 
voltage  constant  current,  or  series  working,  separate  ex- 
citation in  other  high-voltage  work,  and  shunt  machines, 
for  the  most  part,  in  constant-potential  multiple  work. 
Separate  excitation  is  the  most  flexible  method,  in  that 
it  affords  a  ready  means  of  securing  any  combination  of 
amperes  and  turns  to  make  up  a  given  number  of  ampere 
turns.  For  a  given  number  of  ampere  turns  (usually  written 


284 


TESTING    OF    DYNAMOS    AND    MOTORS. 


Si}  the  same  amount  of  work,  or  watts,  is  expended  in 
maintaining  a  field,  whether  the  winding  is  coarse  or  fine. 
Suppose,  for  instance,  that  around  a  spool  of  10  turns, 
measuring  i  ohm,  there  flows  a  current  of  10  amperes. 
If  we  halve  the  cross-section  of  the  wire  we  can  get  on 
twice  as  many  turns,  and  it  will  require  but  5  amperes  to 
give  the  same  Si  as  before.  But  in  halving  the  cross- 
section  we  double  the  resistance  of  every  turn  of  wire, 
and  since  we  have  doubled  the  number  of  turns  we  double 
again  the  resistance  of  the  wire  on  the  spool.  In  other 
words,  we  have  not  only  doubled  the  length,  but  also 
halved  the  cross-section.  Tabulating  this  we  get: 


Turns. 

Amps. 

Amp.  -Turns. 

Res.-Ohms. 

Watts    Lost. 

First  Case 

10 

10 

IOO 

i 

(/'2  J?)—  IOO 

Second  Case.  .  .  . 

20 

5 

100 

(/   A})  =  ioo 

Now  while  it  makes  no  difference  so  far  as  the  magneti- 
zation is  concerned  whether  the  Si  are  made  up  of  many 
turns  and  small  current,  or  vice  versa,  it  does  make  a  dif- 
ference electrically  where  a  machine  is  to  furnish  its  own 
exciting  current.  _  If  in  a  shunt  dynamo  the  field  winding 
is  of  the  right  number  of  turns  but  of  too  low  resistance, 
it  will  take  more  current  than  it  ought  to,  but  will  not 
raise  the  machine's  voltage,  and  hence  the  output,  pro- 
portionally. Properly  designed  shunt  windings  are  of 
such  resistance  as  to  let  in  that  current  which  will  mag- 
netize the  iron  to  best  saturation,  and  any  current  above 
this  over-saturates  the  fields,  reduces  the  number  of  lines 
of  force  per  ampere,  and  brings  down  the  efficiency  of 


SHUNT    AND    COMPOUND    MACHINES.  285 

the  machine.  Sir  William  Thomson  (Lord  Kelvin)  has 
pointed  out  that  no  shunt  dynamo  can  run  at  an  efficiency 
of  90$  unless  its  field  resistance  is  at  least  324  times  that 
of  the  armature.  Without  repeating  the  mathematical 
work  presented  by  both  Sir  William  Thomson  and  S.  P. 
Thompson,  we  will  pass  directly  to  the  more  practical 
deductions  which  they  have  developed  as  regards  the  re- 
sistance of  different  parts  of  the  circuit,  and  its  influence 
upon  the  efficiency.  First,  for  all  practical  work  the  effi- 
ciency (//)  is  given  nearly  enough  by  the  formula, 

i 


Second.  The  most  economical  value  of  the  external  re- 
sistance to  work  with  is  given  by  the  formula  A'  = 
V'"s  '"a  »  where  ra  and  r&  are  respectively  the  field  and 
armature  resistances.  Suppose  we  wish  a  machine  to 
have  an  efficiency  of  90$,  what  must  be  the  relative  re- 
sistances of  armature  and  field?  Substituting  in  the 
above  formula  we  get 


9       " 


V7'-.  Vr. 

&. 


Whence  clearing  of  fractions, 

.9  Vrs  +  i.Sy^  =    ^  and  1/78  -  .9V^8  =  1.8  W7 
or  .1  Vrs  =  1.8  Vr&.       Multiplying    through  by  10  to  get 
rid    of   the    decimals    we   have     ^    -  18  y^a 

•'•    >'s   =    (iS)Va   =    324  ra. 


286  TESTING    OF    DYNAMOS    AND    MOTORS. 

For  any  other  proposed  efficiency  it  is  only  necessary  to 
substitute  and  go  through  a  similar  operation  to  get  the 
relation  which  rs  must  bear  to  r&.  As  a  practical  ex- 
ample, let  1\  —  .206  :  what  value  of  rs  will  secure  a 
maximum  efficiency  of  95^,  and  of  90$  respectively? 
The  equation  is 

_  i 

~ 
which  gives 

.-= 


as  shown  above  in  the  determination  of  the  best  relation 
of  ra  and  rs  for  ;;  =  90^.  Clearing  this  equation  of  frac- 
tions we  have  :  t?  Vrs  -j-  27;  yVa  =  \frs  ;  transposing, 
Vrs  —  11  \/ra  =  2?7\/ra!  factoring  on  Vrs  ,  Vra  (i  —  ^/) 
—  277  |/;'a  •  Dividing  both  sides  by  i  —  77  , 

&  =  -^. 

Krs  1—77 

and  substituting  in  this  formula  for  rj  and  ra,  their  re- 
spective values  .95  and  .206  ohm,  we  get 

2  X  -95  V-  206         1-90  X    4/^06 


or  Vrs  —  38  t/  206,  and  rs  (squaring  both  sides)  =  1,444 
X  .206  =  297  ohms.  For/;  =  90^  =  .9,  rs  =  324  X  .206 
=  66.74  ohms.  As  a  practical  example  of  the  second 
case,  let  r&  =  034  ohms,  and  rs  =  13.6  ohms,  what  is 
the  best  value  for  R  ?  We  have  R  =  V~r*r* 
=  V  13.6  x  .034  =  .68  ohm.  From  a  consideration  of 


SHUNT    AND    COMPOUND    MACHINES.  287 

the  two  problems  in  Case  i,  we  see  that  to  secure  a  5$ 
increase  in  efficiency  the  magnet  resistance  must  be 
raised  from  66  to  297  ohms,  and  this  means  a  great  in- 
crease in  cost  of  production.  Practice  compromises  be- 
tween first  cost  and  efficiency.  Let  us  suppose  the 
resistance  of  a  given  set  of  magnets  to  be  200  ohms. 
What  must  r&  be  to  give  an  efficiency  of  oo^?  Of  95$? 
Taking  the  equation  A//'S  (i  —  ty)  =  2//  f /tt,  and  dividing 
off  by  2;;,  we  have, 

V^.  ('  --  v) 
~ST 

Substitute  for  /;  and  ra  their  respective  values, 

<\/200   (i    --    .QO)  <\/200    X.I  I 

Vr&~-  -j-jp  —  -8-        --V*oox^ 

Whence 


-(*)• 

for  90$  efficiency,  and  for  95$ 


3 

'  ''KXD)  =  200  X  -  -  =  .6172  ohm, 
324 


200 

—  =  .  iios  ohm. 
,444 

Here  to  secure  5$  increase  in  efficiency,  ra  must  be  low- 
«red  to  less  than  one-fourth  of  its  value  for  90$  effi- 
ciency. It  is  not  easy  to  vary  the  resistance  of  an 
armature  intended  to  do  a  certain  amount  of  work, 
unless  it  so  happens  that  several  points  in  bad  design 
may  be  changed  to  right  matters.  If  there  is  no  such 
outlet  the  machine  must  be  either  redesigned  or  rated 
lower.  Field  resistance  should  be  made  as  high,  and 
armature  resistance  as  low,  as  possible. 


288  TESTING    OF    DYNAMOS    AND    MOTORS. 

As  shunt  dynamos  are  not  perfectly  self-regulating  for 
constant  potential,  some  method  of  regulation  must  be 
used;  the  universal  method  is  to  place  a  variable  resist- 
ance (rheostat)  in  the  field  circuit  so  that  its  resistance, 
and  hence  field  current,  can  be  .varied  to  meet  the  de- 
mand. As  we  have  seen  in  a  shunt  machine  with  closed 
switch,  the  current  leaving  the  armature  takes  two  paths; 
the  field  winding  and  external  circuit,  and  the  current 
through  each  depends  upon  their  relative  resistances  un- 
less the  potential  difference  is  kept  constant,  in  which 
case  the  current  in  each  depends  upon  its  individual  re- 
sistance. If  at  any  given  load  the  mains  are  found  to  be 
at  the  proper  potential  difference,  it  means  that  the  field 
circuit  resistance  lets  in  just  field  current  enough  to 
maintain  the  proper  field  strength.  If  R  is  increased  or 
diminished,  by  diminishing  or  increasing  the  load,  the 
rheostat  must  be  turned  to  restore  the  potential  to 
its  former  value.  As  the  load  increases,  or  what  is  the 
same  thing,  as  R  decreases  the  potential  difference  at  the 
brushes  decreases,  and  the  rheostat  resistance  must  be 
reduced  until  the  increased  field  current  provides  volt- 
age enough  to  look  after  the  increased  drop  in  the  arma- 
ture. If  the  external  resistance  becomes  too  low,  as  in 
the  case  of  a  short  circuit  on  the  line,  the  machine  will 
lose  its  field,  if  the  suddenly  precipitated  load  does  not 
open  a  circuit  breaker  or  throw  the  belt  first,  and  will 
refuse  to  "pick  up"  a  field  until  the  short  circuit  is 
removed.  This  is  because  when  a  short  circuit  occurs 
on  the  line,  the  low  resistance  of  the  short  circuit  is  in 
multiple  with  the  high  resistance  of  the  field  circuit,  and 
prevents  the  latter  from  getting  any  current:  or  more 
properly  speaking  the  armature  "drop"  becomes  so 


SHUNT    AND    COMPOUND    MACHINES.  289 

great  that  there  is  not  left  across  the  field  terminals 
potential  difference  enough  to  support  the  field.  In  such 
a  case  the  line  switch  must  be  opened,  and  the  trouble 
sought  on  the  machine  itself.  We  specify  line  switch  be- 
cause in  stations  where  many  dynamos  run  in  multiple  it 
is  customary  to  keep  the  head  board  switch  closed,  and 
to  do  all  testing  across  the  switch  on  the  station  switch- 
board. Where  several  shunt  machines  feed  into  the  same 
"  bus  "  bars,  a  short  circuit  on  the  outside  is  apt  to  make 
them  all  lose  their  fields,  but  in  some  cases  the  fault 
burns  out  as  soon  as  it  is  made,  while  in  others  special 
preparations  must  be  made  for  burning  it  out.  This  is 
treated  of  elsewhere,  as  is  also  the  special  case  of  short 
circuit  in  the  machine  itself  as  a  consequence  of  a  broken 
field  wire. 

If  a  shunt  machine  has  most  of  its  load  suddenly 
removed,  by  cutting  out  parts  of  the  service,  the  volt- 
age will  rise  and  endanger  the  lives  of  the  lamps  still 
in  circuit.  This  is  because  at  the  larger  load  the  arma- 
ture drop  is  the  greater,  and  the  field  current  is  adjusted 
to  look  after  it.  When  the  load  is  removed  the  arma- 
ture drop  is  much  less,  leaving  a  greater  potential  differ- 
ence for  the  external  circuit  whose  resistance  has  been 
increased.  The  immediate  effect  of  raising  the  voltage 
at  the  field  terminals  is  to  send  a  larger  current  through 
the  winding,  thereby  again  sending  the  voltage  upward. 
This  action  is  most  marked  where  there  is  but  a  single 
dynamo  in  service,  the  reason  being  that  where  several 
dynamos  are  in  multiple,  the  decrease  in  load  is  divided 
among  them  all,  and  the  amount  per  machine  is  not  so 
large.  The  remedy  is  to  cut  resistance  into  the  field 
rheostat.  This  liability  to  injure  lamps  is  one  reason. 


290  TESTING    OF    DYNAMOS    AND    MOTORS. 

aside  from  the  desire  to  keep  lamps  at  constant  brilliancy, 
that  the  attendant  watches  so  closely  at  that  time  in  the 
evening  when  a  great  many  lamps  are  being  turned  out. 
Turning  off  all  the  lamps  in  a  public  building  will  very 
perceptibly  affect  the  voltage  of  a  single  dynamo.  An 
attendant  soon  comes  to  know  the  unsteady  times  of  the 
day  and  keeps  a  hand  near  the  rheostat.  Where  the  ser- 
vice is  such  that  load  variations  are  not  sudden  or  great, 
an  attendant  is  equal  to  the  occasion,  otherwise  the  shunt 
machines  had  better  give  way  to  compound-wound 
machines,  whose  rheostats  are  set  once  for  all  and  re- 
quire no  further  attention. 

The  extent  to  which  a  given  change  in  the  rheostat 
resistance  will  affect  the  dynamo's  voltage  depends  upon 
the  relative  resistance  of  the  field  winding  and  rheostat, 
and  this  relation  in  turn  depends  upon  their  temperatures. 
It  must  be  borne  in  mind  that  the  two  are  usually  com- 
posed not  only  of  different  substances,  but  have  very 
different  facilities  for  radiating  heat,  so  that  a  given  cur- 
rent will  raise  their  resistances  at  quite  different  rates. 
In  winter  when  the  surrounding  air  may  be  at  15°  C.  to 
20°  C.,  it  might  be  necessary  to  use  all  of  the  rheostat  to 
hold  the  voltage  at  its  proper  value,  while  in  summer 
with  the  temperature  of  the  air  surrounding  field  and  box 
at  70°  C.  or  80°  C.,  the  rheostat  must  be  almost  cut  out, 
the  increased  resistance  of  the  field  sufficing.  For  the 
same  reason  a  rheostat  adjustment  when  the  field  is  cold 
is  of  no  value  when  the  field  *a  r  loss  has  raised  the  field 
temperature,  and  with  it  its  resistance.  Further,  the 
armature  heats  under  load,  and  its  increased  resistance 
causes  a  greater  internal  "drop"  through  it;  and  finally, 
after  heating  up,  the  iron  or  steel  parts  of  the  machine 


SHUNT    AND    COMPOUND    MACHINES.  29! 

do  not  carry  lines  of  force  so  well,  or  as  we  say,  the 
reluctance  of  the  magnetic  circuit  has  increased,  so  that 
it  takes  more  magnetizing  force  to  produce  a  given 
amount  of  magnetization.  All  of  these  causes  call  for 
more  ampere-turns  on  the  field,  to  keep  up  the  voltage, 
and  since  we  cannot  in  most  cases  increase  the  turns,  we 
must  use  a  rheostat  to  increase  the. amperes,  and  this 
device  is  necessary  aside  from  its  use  in  regulating  varia- 
tions due  to  changes  in  load.  True,  minor  variations 
can  be  cared  for  by  rocking  the  brushes,  forward  to  raise 
the  voltage  and  backward  to  lower  it,  but  since  there  is 
always  a  best  position  for  the  brushes,  other  positions  are 
not  so  good,  and  for  several  reasons  the  practice  is  con- 
demned. One  reason  is  the  effect  upon  the  efficiency. 
The  brushes,  although  apparently  not  sparking,  really  do, 
and  hence  heat  more.  One  other  reason,  to  be  dealt  with 
later,  is  that  the  regulation  depends  largely  upon  arma- 
ture reaction,  and  this  means  that  a  certain  amount  of 
the  field's  magnetism,  which  costs  money,  is  being  neu- 
tralized. 

One  must  not  be  deluded  into  the  idea  that  a  rheostat 
can  of  itself  raise  a  dynamo's  voltage,  for  when  the 
rheostat  is  short  circuited  it  might  just  as  well  be  out  of 
circuit.  This  is  a  very  delusive  error,  wanting  thought, 
and  when  sustained  by  circumstances  is  misleading. 
The  writers  recall  one  instance  where  two  young  men 
were  using  a  machine  running  from  a  line  of  heavily 
loaded  shafting,  turning  below  speed  because  the  steam 
pressure  was  low.  They  were  imbued  with  the  idea  that 
the  voltage  was  generally  brought  up  by  cutting  the 
rheostat  out,  so  not  having  one  in  they  decided  to  put 
one  in  so  they  could  cut  it  out.  Acting  upon  the  resolu- 


292  TESTING    OF    DYNAMOS    AND    MOTORS. 

tion  they,  put  one  in.  In  the  meanwhile  the  speed  of  the 
shaft  had  risen  10  revolutions,  raising  that  of  the  dynamo 
about  25  revolutions.  Of  course  the  voltage  was  raised 
and  they  were  satisfied  that  the  rheostat  was  the  respon- 
sible party. 

In  changing  the  field  rheostat  resistance  on  a  dynamo 
two  effects  obtain,  and  these  are  best  studied  by 
considering  a  separately  excited  machine,  for  in  this 
case  the  two  can  be  separated.  In  Fig.  104,  A  is  an 


FIG.  104. 

armature,  and  F  a  field,  separately  excited  from  the 
battery  or  exciter  B.  R  is  a  variable  resistance  rheostat, 
in  series  with  F,  but  in  no  way  connected  with  A.  This 
arrangement  typifies  a  separately  excited  machine.  Now 
when  switch  K  is  open  no  current  flows  in  F,  and  what 
voltage  can  be  measured  on  A  is  due  solely  to  the  residual 
magnetism,  and  we  will  suppose  that  by  some  means  or 
other  this  is  gotten  rid  of,  so  that  until  K  is  closed  A 
generates  no  voltage.  Next  close  K,  and  suppose  A 
generates  500  volts.  R  is  cut  out,  and  the  current 
through  F\<*>  that  which  B  will  send  through  its  resistance. 
Now  increase  R  until  its  resistance  equals  that  of  F:  we 
have  halved  the  field  current  because  we  have  doubled 
the  field  circuit  resistance,  the  voltage  of  B  having 


SHUNT    AND    COMPOUND    MACHINES.  293 

remained  the  same.  In  halving  /,  the  exciting  or  mag- 
netizing power  or  ampere-turns,  Si,  have  been  halved, 
and'  A  is  found  to  generate  250  volts — its  speed  assumed 
to  be  the  same.  If  the  total  resistance  of  F's  circuit  be 
again  doubled,  the  voltage  will  fall  to  the  neighborhood 
of  125  volts.  We  cannot  say  exactly  in  any  case,  because 
the  effect  which  any  given  increase  in  field  current  has, 
depends  upon  how  nearly  saturated  the  iron  frame  is,  but 
since  this  error  will  occur  in  both  cases  it  will  not  affect 
our  comparison.  Better  still  let  us  suppose  the  machine 
has  no  magnetic  metal  in  it,  and  the  above  figures  become 
true,  and  we  can  say  that  A's  voltage  is  strictly  pro- 
portional to  F's  current  and  inversely  proportional  to 
(F  -|-  Ry?>  resistance,  that  is,  if  the  field  circuit  resist- 
ance is  halved,  A's  voltage  is  doubled;  if  it  is  doubled, 
A's  voltage  is  halved.  Now  suppose  B  to  be  removed, 
and  the  shunt  machine  connected  to  excite  itself  (the 
machine  would  never  build  up  a  field  through  a  magnetic 
circuit  having  no  iron  or  steel  in  it,  but  this  will  not  affect 
our  demonstration).  Further  suppose  R  to  be  such  that 
A  gives  a  voltage  of  500:  now  vary  R  so  the  field  circuit 
resistance  (F-\-  R)  is  doubled.  In  the  separately  excited 
machine  varying  R  did  not  affect  the  exciting  voltage, 
because  B  was  independent  of  all  changes  in  A's  voltage, 
but  in  this  case,  as  soon  as  R  is  increased  the  voltage  on 
A  decreases  as  before;  but  now  A's  voltage  excites  F  so 
that  not  only  is  the  field  circuit  resistance  increased,  but 
the  exciting  E.  M.  F.  is  also  decreased:  both  changes  con- 
spire to  reduce  A's  E.  M.  F.,  and  if  R  is  increased  enough 
the  machine  will  drop  its  field  entirely.  We  may  say  then, 
that  qn  a  shunt  machine  a  given  change  in  the  rheostat 
has  a  greater  effect  than  on  a  separately  excited  machine. 


294  TESTING    OF    DYNAMOS    AND    MOTORS. 

The  presence  of  the  rheostat  is  sometimes  referred  to 
as  objectionable,  in  that  it  introduces  an  additional  /V 
loss  into  the  equation  for  efficiency.  While  this  is  in  a 
certain  sense  true,  it  can  only  be  avoided  by  winding  the 
fields  with  an  unheard-of  metal  whose  resistance  is  the 
same  for  all  temperatures,  and  doing  away  with  all  arma- 
ture drop.  If  the  former  could  be  done  there  would 
remain  the  variation  of  E.  M.  F.  due  to  variation  of  load, 
and  could  the  latter  difficulty  be  overcome  there  would 
still  be  a  great  difference  between  the  voltages  for  hot  and 
cold  fields.  It  must  be  remembered  that  for  a  specified 
voltage  at  the  terminals  there  must  be  a  specified  current 
in  the  field  circuit,  which  must  therefore  be  of  a  specified 
resistance.  The  ampere-turns  necessary  to  maintain  the 
proper  potential  for  any  given  load  is  independent  of 
temperature  as  far  as  concerns  the  windings  of  the  field, 
and  depends  only  upon  speed  («),  the  number  of  armature 
conductors  (C),  and  the  goodness  of  the  magnetic  circuit. 
But,  as  we  have  seen,  the  temperature  influences  the 
ampere-turns  indirectly  by  raising  the  armature  resist- 
ance, and  increasing  its  /V  loss.  Assuming  the  machine 
to  be  running  under  constant  load,  so  that  the  rheostat 
will  not  have  to  regulate  load  variations,  and  that 
the  potential  at  the  brushes  is  to  be  kept  constant, 
there  is  a  certain  field  circuit  resistance  which  corre- 
sponds to  a  particular  value  of  the  ampere-turns,  and  this 
resistance  is  the  same  whatever  the  temperature,  except 
in  so  far  as  the  temperature  affects  the  permeability  of 
iron  or  steel.  It  matters  not,  as  far  as  concerns  the 
dynamo's  electric  efficiency,  whether-  this  resistance  is 
found  in  the  winding  alone,  as  it  often  is  in  hot  weather, 
or  in  winding  and  rheostat  combined,  as  in  cold  weather. 


SHUNT    AND    COMPOUND    MACHINES.  295 

The  7V  loss  is  a  constant,  and  whether  or  not  it  is 
divided  between  field  and  rheostat  is  immaterial.  How- 
ever, heating  in  itself  is  guarded  against  in  well-designed 
machines,  for  excessive  heating  carbonizes  the  insulation 
and  ultimately  gives  rise  to  serious  trouble.  Since  heat 
is  in  any  case  generated  at  the  rate  of  7V  watts  per 
second,  the  temperature  will  rise  rapidly,  unless  there  are 
provided  proper  facilities  for  ventilation.  This  means 
practically  that  the  winding  must  have  ample  surface  for 
radiation.  The  temperature  limit  for  ordinary  machines 
may  be  set  at  50°  C.  above  that  of  the  surrounding  air, 
but  on  inclosed  motors  of  the  street  railway  type,  the 
temperature  rises  above  this,  and  on  machines  of  special 
design,  as  in  the  Marvin  rock  drill,  where  the  magnets 
are  mica  insulated,  the  temperature  may  reach  much 
higher. 

To  secure  the  50°  C.  limit  there  should  be  2  1/2  square 
inches  of  radiating  surface  for  every  watt  of  energy 
wasted.  Suppose,  for  instance,  a  magnet  spool  of  i 
ohm  resistance  takes  30  amperes  under  full  load.  Watts 
wasted  —  7V  —  I  x  I  X  /'  —  30  X  30  x  i  —  900  watts; 
900  x  2  1/2  square  inches  =  2,250  square  inches  of  surface 
required  to  be  exposed  to  the  air.  In  the  Brush  machine, 
2  square  inches  are  allowed  the  fields,  and  9  square  inches 
per  watt  waste  in  the  armature;  in  the  T.-H.  armature 
i  2/3  square  inch  is  allowed.  These  figures  illustrate 
the  fact  that  the  rule  is  seldom  adhered  to;  in  other 
words,  machines  are  in  most  cases  allowed  to  run  at  a 
temperature  exceeding  50°  above  the  atmosphere.  Note 
that  less  radiating  surface  is  allowed  on  armatures  than 
on  fields;  this  is  because,  as  a  rule,  field  wires  are  wound 
many  layers  deeper  than  armature  wires,  and  besides  this 


296  TESTING    OF    DYNAMOS    AND    MOTORS. 

the  armature  is  fanned  by  the  current  of  air  produced 
by  its  own  motion.  In  general,  the  2  1/2  square  inch  rule 
applies  to  machines  run  continuously  at  full  load,  and  as 
this  condition  is  not  ordinarily  required  to  be  fulfilled, 
the  rule  is  not  rigidly  adhered  to.  The  main  reason  for 
the  reduction  in  radiating  surface  has  been  the  desire  to 
make  the  machine  lighter  and  smaller,  and  it  is  brisk 
competition  that  has  forced  the  figure  so  low. 

To  understand  how  increasing  the  radiating  surface 
also  increases  a  machine's  weight,  let  us  suppose  that  we 
have  a  field  winding  that  is  running  too  hot.  To  increase 
the  square  inch  of  radiating  surface  per  watt,  we  must 
either  arrange  the  layers  of  wire  to  lie  less  deep,  but 
wider,  or  we  must  decrease  the  number  of  watts.  In  the 
first  case  the  pole  piece  or  magnet  core  must  be  lengthened, 
and  with  it  necessarily  every  part  of  the  frame  parallel 
to  the  direction  of  elongation.  In  the  second  case  (in- 
directly the  same  as  the  first)  a  larger  wire  must  be  used, 
and  in  order  to  get  in  the  same  number  of  turns  as  before, 
more  room  is  required.  This  gives  more  surface,  and  we 
have  not  reduced  the  number  of  watts  wasted,  but  we  have 
reduced  the  watts  per  square  inch.  The  same  number 
of  turns  of  a  larger  size  wire  would  be  of  too  low  a 
resistance,  and  would  let  in  too  great  a  current,  so  a  larger 
resistance  must  be  inserted  in  the  rheostat.  In  cases 
where  a  machine  is  unavoidably  overloaded,  artificial 
cooling  must  be  resorted  to,  otherwise  the  machine  must 
be  shut  down  at  intervals  to  cool  off.  The  writers  recall 
one  instance  of  where  a  street  railway  waterproof  motor 
attached  to  the  machine  shop  shafting  was  kept  cool  by 
allowing  a  stream  of  water  to  play  on  it.  In  another 
case  reversed  fans  were  placed  on  either  side  of  a  dynamo 


SHUNT    AND    COMPOUND    MACHINES.  297 

to  draw  a  strong  current  of  air  through  it.  Shunt  wind- 
ings are  so  designed  that  at  no  load  the  rheostat  is  called 
largely  .into  play,  and  as  the  load  goes  on,  the  ampere- 
turns  can  be  increased  to  suit  the  demand  by  taking 
resistance  out  of  the  rheostat.  In  winter,  and  with  full 
load,  there  is  still  enough  resistance  in  the  box  to  equal 
the  amount  by  which  the  fields  will  need  to  be  raised  in 
summer. 

Field  rheostats  are  generally  made  of  German  silver 
wire,  which  rises  to  a  higher  temperature  for  a  given 
current  than  copper,  but  its  resistance  does  not  rise 
nearly  so  much  per  degree  rise  in  •temperature.  Since 
its  specific  resistance  is  greater,  not  so  much  wire  is 
needed  to  make  up  a  given  resistance,  and  German  silver 
rheostats  are  therefore  lighter  and  less  bulky  than  those 
of  iron  or  copper.  The  rheostat  should  be  placed  where 
ventilation  is  good,  and,  if  properly  made,  will  heat  but 
little.  Rheostats  are  shipped  with  each  machine.  In 
adapting  a  box  for  use  as  a  rheostat,  care  must  be  taken 
that  not  only  the  desired  resistance  is  had,  but  that  the 
wire  is  of  sufficient  current  carrying  capacity,  otherwise 
the  heating  will  be  abnormal,  and  a  connection  may  be 
melted  off,  thereby  perhaps  opening  the  field  circuit  on  a 
machine  running  in  multiple  with  others.  We  recall  one 
test  in  which  it  was  necessary  to  get  along  with  a  rheostat 
of  insufficient  current  capacity;  so  we  put  a  similar  one 
in  series  with  it,  and  kept  cutting  one  out  at  the  same 
rate  as  we  cut  the  other  in,  and  in  this  way  relieved  first 
one  and  then  the  other.  Had  the  boxes  been  put  in 
multiple  their  current  capacity  would  have  been  doubled, 
but  there  would  not  have  been  enough  resistance.  The 
best  arrangement  would  have  been  two  in  series  and  two 


298 


TESTING    OF    DYNAMOS    AND    MOTORS. 


in  multiple.  This  would  give  the  resistance  of  one  box 
and  the  current  capacity  of  two  in  multiple.  But  only 
two  boxes  were  to  be  had. 

Shunt  machines  can  be  run  either  in  series  or  parallel, 
but  are  specially  adapted  to  the  latter  practice.  When 

run  in  series  it  is  usually 
on  a  three-wire  lighting  or 
power  system.  Sometimes 
two  low-voltage  machines 
are  connected  in  series, 
and  the  combination  run  in 
multiple  with  a  machine  of 
twice  the  voltage  of  either 
— as  when  two  250  volt 
dynamos  in  series  are  run 
in  multiple  with  a  500 

Flo   I05  volt  machine.    Connections 

are  shown  in  Fig.  105.     In 

such  a  case  the  current  flowing  through  the  two  in  series 
must  not  exceed  the  capacity  of  the  smaller  machine. 
The  current  in  both  being  the  same,  each  one's  load  will 
be  proportional  to  its  voltage,  and  can  be  regulated  by 
means  of  the  latter.  It  is  customary  to  connect  the 
two  fields  in  series  across  the  500  volt  "bus  "bars.  A 
single  rheostat,  if  of  sufficient  range,  answers  for  reg- 
ulation. 

Where  there  is  a  single  shunt  dynamo  on  a  circuit,  it  is 
customary  to  connect  the  fields  on  the  armature  side 
of  the  open  switch,  or,  as  it  is  usually  said,  "  below  the 
switch,"  so  that  the  field  may  be  excited  before  the  switch 
is  closed.  Very  often  in  single  dynamo  private  installa- 
tions the  switch  is  never  opened  nor  the  field  rheostat 


SHUNT    AND    COMPOUND    MACHINES.  299 

changed  from  one  month's  end  to  the  other.  The  load 
being  put  on  and  taken  off  by  starting  up  and  shutting 
down  the  engine.  When  several  machines  are  run  in  mul- 
tiple the  fields  are  connected  above  the  switch,  or  direct 
to  the  "  bus"  bars,  thus  in  a  measure  separately  exciting 
the  machine,  since  it  has  a  field  whether  running  or  not. 
As  soon  as  the  machine  contributes  its  share  of  work  to 
the  line,  it  may  be  regarded  as  exciting  itself.  Ordina- 
rily this  arrangement  insures  that  the  machine's  polarity 
is  such  that  when  the  switch  is  closed  the  machine  will 
be  in  multiple  and  not  in  series  with  the  machines  already 
in  service.  However,  because  a  machine's  field  is  charged 
from  the  line,  it  does  not  necessarily  follow  that  the  po- 
larity may  not  be  wrong,  for  it  is  possible  that  the  field 
connections  may  be  wrong.  In  such  a  case  the  machine 
and  the  line  will  be  in  series  and  with  no  resistance  in 
circuit  save  that  of  the  armatures — a  situation  by  all 
means  to  be  avoided.  Promiscuous  line  charging  does 
not  of  course  insure  proper  polarity,  and  the  fact  should 
be  tested  before  closing  the  line  switch.  The  test  is  as 
follows:  Connect  the  fields  below  the  switch,  raise  the 
•brushes  (or  where  there  are  many  sets  of  brushes,  discon- 
nect an  armature  cable)  and  close  the  switch,  thus 
charging  the  fields  from  the  line.  Now  open  the  switch. 
The  field  circuit  is  broken,  but  the  poles  retain  a  con- 
siderable a.nount  of  residual  magnetism.  Lower  the 
brushes  and  observe  if  the  dynamo  supports  its  own 
field;  if  so,  the  polarity  is  correct,  because  charging  from 
the  line  insures  proper  polarity  if  the  connections  are 
correct,  and  if  they  are  not  correct  the  machine  will  not 
support  its  field.  Assuming  that  the  machine  generates, 
the  connection  which  was  below  the  switch  can  be  per- 


300  TESTING    OF    DYNAMOS    AND    MOTORS. 

manently  secured  above  it.  If  it  refuses  to  generate  it 
means  that  the  armature  E.  M.  F.  is  such  as  to  send  a 
reverse  current  through  the  fields  opposing  the  residual 
field  due  to  charging.  In  other  words,  the  field  leads  are 
connected  wrong  and  must  be  reversed,  and  the  fields 
recharged.  This  point  is  more  fully  considered  in  a  later 
chapter. 

The  fact  that  the  fields  of  shunt  machines  are  independ- 
ent of  the  external  circuit  adapts  them  for  running  in 
multiple  on  constant  potential  mains.  To  run  dynamos 
in  multiple  successfully  it  is  desirable  that  they  be  of  the 
same  rated  E.  M.  F.,  fora  machine  will  not  always  behave 
well  when  run  at  other  than  its  designed  voltage,  and 
careless  handling  of  the  brushes  or  rheostat  is  apt  to 
reverse  the  machine,  running  it  as  a  motor  with  destruc- 
tive sparking.  The  extent  of  sparking  depends  upon 
how  much  load  is  on  the  machine  at  the  time  of  reversal. 
For  a  dynamo,  on  heavy  load,  the  non-sparking  point  is 
well  forward.  A  motor  under  similar  conditions  has  this 
point  well  backward,  so  that  in  reversing  from  dynamo  to 
motor  the  non-sparking  point  is  shifted,  and  unless  the 
brushes  are  shifted  also,  sparking  ensues.  Aside  from- 
sparking  no  bad  effects  attend  a  shunt  machine's  reversal 
of  nature,  for  it  continues  to  run  in  the  same  direction  as 
a  motor  that  it  did  as  a  generator.  The  behavior  of 
series  and  compound-wound  machines  under  like  condi- 
tions has  already  been  briefly  considered.  We  will  now 
consider  the  question  more  fully,  and  explain  the  facts 
that  (i)  a  shunt  machine  rotates  the  same  way  as  a  motor 
and  as  a  generator;  (2)  a  series  machine  rotates  in  oppo- 
site directions  in  the  two  cases;  (3)  a  compound-wound 
machine's  behavior  when  changed  from  dynamo  to  motor 


SHUNT    AND    COMPOUND    MACHINES.  301 

depends    upon  the   relative   strength   of    the   shunt  and 
series  windings. 

When  a  machine  runs  as  a  dynamo  the  source  of  cur- 
rent is  within  the  armature;  when  run  as  a  motor  cur- 
rent is  supplied  from  without.  Whether  a  machine  will 
run  in  the  same  or  opposite  direction  as  motor  and 
dynamo  is  independent  of  the  direction  of  the  current  in 
the  external  circuit,  and  depends  solely  upon  the  relation 
existing  between  the  current  direction  in  the  armature 
and  that  in  the  field.  On  a  shunt  dynamo  it  matters  not 
which  brush  the  current  flows  toward,  it  must  eventu- 
ally divide  between  the  line  and  the  field  circuit.  The 
field  circuit  shunts  the  line.  If  on  a  shunt  dynamo  we 
assume  that  field  and  armature  currents  flow  in  opposite 
directions  as  regards  a  point  in  space,  they  will  always  do 
so.  Reverse  the  machine's  polarity,  if  you  will,  by 
reversing  its  residual  field.  Current  through  arma- 
ture and  field  both  reverse  direction,  and  therefore  are 
still  opposed  as  regards  a  point  in  space.  Now  regard 
the  machine  as  a  motor:  current  from  without,  whatever 
its  direction,  must  divide  between  armature  and  field. 
The  field  circuit  now  shunts  the  armature,  and  it  is  impos- 
sible to  introduce  current  from  outside  and  have  it  go 
through  field  and  armature  in  opposite  directions  as  re- 
gards a  point  in  space,  the  connections  being  the  same. 
In  a  shunt  machine,  reversing  from  dynamo  to  motor, 
the  relative  direction  of  armature  and  field  currents  is 
changed,  because  the  armature  current  reverses,  while 
the  field  current  does  not.  This  is  true,  for  suppose  the 
machine  to  be  running  as  a  motor;  the  armature  has  a  C. 
E.  M.  F.,  and  this  counter  tries  to  send  through  the  arma- 
ture a  current  opposed  to  that  urged  by  the  impressed  or 


/ 


302  TESTING    OF    DYNAMOS    AND    MOTORS. 

line  E.  M.  F.  If  the  impressed  E.  M.  F.  is  removed  and 
the  armature  kept  turning  by  some  other  means,  the  C.  E. 
M.  F.  will  assert  itself  as  the  E.  M.  F.  of  the  machine  run- 
ning as  a  dynamo,  and  will  dictate  the  direction  of  the 
current.  This  will  be  more  clear  by  considering  Fig.  106. 

B  B* ',  M,  and  F  are  re- 
spectively the  brushes, 
armature,  and  field  of 
a  shunt  motor.  A  A' 
and  D  are  the  brushes 
and  armature  of  a 
pia  Io6  dynamo  sending  cur- 

rent  in    the    direction 

indicated  by  arrows.  The  current  arriving  at  B'  splits, 
part  going  through  F,  in  the  direction  indicated  by  the 
arrow,  and  part  up  through  the  armature  M.  The  C.  E. 
M.  F.,  if  allowed  to  assert  itself,  would  send  a  current 
downward  through  J/,  which  at  B'  would  split,  part  going 
through  F,  and  in  the  same  direction  as  before,  and  part 
through  the  external  circuit,  but  in  a  direction  opposite 
to  that  in  which  it  flowed  when  M  was  a  motor.  We  can 
now  say  that  if  in  changing  a  machine  from  dynamo  to 
motor  without  changing  the  connections,  the  relative 
direction  of  the  field  and  armature  currents  is  changed, 
the  rotation  in  both  cases  will  be  the  same. 

In  a  series  machine  armature,  field,  and  line  are  all  in 
series:  whatever  reversal  of  current  takes  place  in  one, 
takes  place  in  all,  so  the  relative  directions  remain 
always  the  same.  A  series  machine  must  then  be  run 
the  opposite  way  as  generator  from  what  it  is  as  motor, 
otherwise  it  will  not  generate  unless  either  the  field  or 
armature  leads  are  reversed. 


SHUNT    AND    COMPOUND    MACHINES.  303 

In  the  compound-wound  machine  the  two  tendencies 
conflict,  the  series  winding  tries  to  reverse  the  polarity 
of  the  field,  the  shunt  winding  to  keep  it  the  same.  The 
stronger  of  the  two  dictates  the  field's  polarity,  and  with 
it  the  direction  of  rotation.  In  a  compound-wound 
machine  connected  to  run  as  a  generator,  current  leaving 
the  armature  flows  around  shunt  and  series  coils  in  the 
same  direction,  thereby  strengthening  the  field  as  load 
goes  on.  If  through  careless  handling  the  machine 
become  reversed,  the  direction  of  current  in  the  series 
winding  is  reversed  while  that  in  the  shunt  is  not.  The 
result  is  that  if  the  windings  are  very  nearly  balanced, 
the  field  is  neutralized,  and  there  is  a  short  circuit.  This 
matter  has  a  bearing  upon  the  operation  of  compound- 
wound  generators  in  multiple.  Ordinarily,  the  equalizer 
prevents  any  troubles  of  this  sort,  but  they  do  occur 
occasionally  in  spite  of  everything,  and  it  is  well  to  be 
forewarned  as  to  what  is  likely  to  happen.  Machines  on 
which  shunt  and  series  windings  are  too  nearly  balanced 
are  apt  to  be  unstable  and  give  trouble.  In  any  event, 
should  such  a  machine  ever  reverse  and  the  circuit 
breaker  fail  to  act,  the  armature  would  race  and  tear 
itself  to  pieces.  We  have  thus  far  had  most  to  do  with 
the  shunt  dynamo  as  adapted  to  use  with  lamps  in 
multiple,  and  we  have  learned  that  the  serious  problem 
of  compensating  armature  drop  has  been  solved  by  the 
use  of  a  rheostat.  Now  this  armature  loss  increases 
very  rapidly  as  the  current  increases,  and  hence  any  step 
that  would  tend  to  decrease  the  current  used  for  a  given 
horsepower  would  decrease  also  the  "lost  watts."  This 
can  be  approximated  by  having  the  lamps  or  other  load  all 
in  series,  and  raising  the  voltage  to  a  high  value ;  if  this  is 


304  TESTING    OF    DYNAMOS    AND    MOTORS. 

done  by  strengthening  the  field  and  running  the  armature 
at  a  higher  speed,  but  holding  the  maximum  current  at 
one  half  its  original  value,  there  is  a  saving  effected  in 
two  ways:  first,  since  the  current  is  halved  there  will  be 
but  one-half  the  original  drop  in  the  armature  at  a  given 
load,  and  since  this  drop  is  distributed  among  so  many 
lamps  in  series, its  effect  will  not  be  so  noticeable;  second, 
it  costs  much  less  to  transmit  a  given  amount  of  energy 
at  high  pressure,  because  a  small  current  occasions 
less  72  R  loss.  If,  however,  the  increased  voltage  is 
secured  by  putting  more  wire  on  the  armature  and 
increasing  its  resistance  proportionately,  there  is  effected 
only  the  saving  in  transmission  as  far  as  drop  is  con- 
cerned, but  there  is  but  half  the  energy  wasted  in  the 
armature,  because  if  for  a  given  armature  the  current 
be  halved,  the  lost  energy  is  but  one-fourth  as  great; 
now  double  the  resistance  and  the  loss  is  doubled  for 
the  same  current,  leaving  the  loss  one-half  as  great.  It 
is  not  practicable  to  raise  the  armature  speed  much 
above  what  it  is  on  ordinary  machines  of  to-day,  and 
the  series  system  is  not  in  general  vogue  for  constant 
potential  lighting.  The  nearest  and  most  successful  ap- 
proach to  it  is  the  "Edison  Municipal  Incandescent 
Lighting  System,"  on  which  there  are  long  lines  of  lamps 
in  ^series,  and  several  of  these  lines  in  multiple.  The 
voltage  most  commonly  used  on  such  a  system  is  1,200 
volts. 

Separately  excited  dynamos  differ  from  self-excited 
ones  in  that  no  attention  need  be  paid  to  the  field 
connections  relative  to  the  armature:  even  where  a 
particular  polarity  is  desired,  a  reversing  switch  in  the 
field  circuit  answers  the  purpose.  If  there  are  several 


SHUNT    AND    COMPOUND    MACHINES.  305 

such  machines  running  in  parallel,  and  one  of  them 
reverses,  its  direction  of  rotation  remains  the  same,  but 
this  is  as  far  as  its  behavior  is  analogous  to  that  of  the 
shunt  machine.  On  a  separately  excited  machine  the  field 
lias  no  connection  with  the  armature,  so  that  when  the 
line  current  is  reversed  the  armature  alone  is  reversed, 
and  not  the  field.  If  the  machine  is  first  a  dynamo  and 
now  a  motor,  the  direction  of  rotation  remains  unaltered; 
but  if  it  is  a  motor  first,  then  under  the  new  conditions 
the  direction  of  rotation  is  reversed.  A  separately 
excited  machine  will  then  run  the  same  way  as  motor 
and  dynamo,  provided  all  machine  and  line  connections 
remain  the  same  and  the  line  polarity  is  not  changed. 

All  four  types  of  machine,  series,  shunt,  compound- 
wound  and  separately  excited,  can  be  run  in  series  or 
multiple,  or  in  any  series  multiple  combination,  provided 
proper  precautions  are  taken.  Machines  of  the  same  type 
run  best  together,  because  their  iron  is  apt  to  be  of  the 
same  quality,  making  their  saturation  curves  similar,  and 
this  means  that  for  any  change  in  load  their  voltages  would 
vary  in  equal  measure,  thus  keeping  the  load  properly 
distributed.  On  similar  machines  the  armature  resist- 
ances are  nearly  the  same,  and  the  armature  drops  will 
be  the  same.  Again,  on  series  and  compound-wound 
machines  it  is,  for  reasons  to  be  considered  later,  abso- 
lutely necessary  that  the  series  field  resistances  bear  a 
certain  ratio  to  each  other  if  the  machines  are  to  be  run 
with  an  equalizer.  On  machines  from  the  same  factory 
this  rule  is  observed.  Machines  of  widely  varying  cur- 
rent capacity  should  not  be  run  in  series,  nor  those  differ- 
ing greatly  in  voltage  in  multiple.  In  series  working 
there  should  be  a  separate  voltmeter  across  the  brushes  of 


306  TESTING    OF    DYNAMOS    AND    MOTORS. 

each  machine  as  a  means  of  noting  the  distribution  of 
load.  The  load  is  regulated  by  varying  the  voltage,  since 
the  current  is  the  same  in  all.  This  voltage  variation  is 
effected  in  the  usual  way  on  a  shunt  machine,  but  on 
a  series  machine  the  rheostat  is  in  multiple  with  the 
field  winding,  and  in  operating  it  more  or  less  exciting 
current  is  diverted  from  the  field.  In  multiple  work  the 
load  distribution  is  indicated  by  ammeters  placed  in  cir- 
cuit with  each  machine.  On  account  of  the  greater 
liability  to  reversals,  more  precautions  are  needed  in 
parallel  than  in  series  work.  One  point  to  be  always 
observed  is  to  never  close  a  switch  across  which  there 
is  a  difference  of  potential,  unless  the  operator  under- 
stands exactly  what  is  to  follow.  Occasions  often  arise 
for  quick  action,  and  it  may  be  necessary  to  throw  a 
machine  in  without  the  usual  preliminaries,  but  this  is 
not  good  practice. 

The  easiest  way  to  throw  a  machine  into  service  where 
others  are  running  in  multiple  depends  upon  the  accuracy 
of  a  zero  reading  across  the  switch.  When  a  voltmeter 
reads  zero  between  two  points,  and  there  is  a  certainty 
that  the  lines  are  all  right  and  make  good  contact,  it  is 
safe  to  join  these  two  points,  for  no  current  can  flow 
between  points  of  the  same  potential.  All  more  hurried 
methods  are  adaptations  of  the  following:  In  Fig.  107, 
let  P  and  N  be  the  line  wires  or  bus  bars  of  a  station,  A 
is  a  shunt  dynamo  already  running,  and  increase  of  load 
requires  B  to  be  put  on.  A  being  in  service  its  switch, 
K,  is  closed,  while  K'  on  the  idle  machine  is  open.  If 
B  is  a  new  machine,  or  one  whose  connections  have  been 
disturbed,  it  will  be  best  to  first  test  its  polarity.  Lift- 
ing .Z?'s  brushes  K'  is  closed,  and  the  fields  changed  from 


SHUNT    AND    COMPOUND    MACHINES. 


307 


the  line.  Then  opening  K'  and  lowering  the  brushes  the 
machine  will  pick  up  a  field  if  the  connections  are  cor- 
rect. This  test  being  satisfied,  one  voltmeter  terminal  is 
held  on  the  right  hand  block  b'  on  the  headboard  of  the 
machine,  and  the  other  terminal  is  touched  to  the  upper 
switch  block  on  the  left.  The  reading  gives  the  line 
voltage.  Transferring  the  meter  terminal  from  the 


FIG.  107. 

t 

upper  jaw  to  the  lower,  we  get  the  machine's  voltage, 
which  must  be  so  adjusted  by  means  of  the  rheostat  that 
it  equals  that  of  the  line.  Whc'n  these  voltages  are  equal 
and  opposite  the  meter  will  indicate  zero  when  placed 
across  the  switch,  for  the  line  voltage  tries  to  send  cur- 
rent through  it  one  way,  and  &s  voltage  the  opposite; 
the  tendencies  are  equal,  so  the  needle  does  not  move. 
At  this  point  the  switch  may  be  closed,  and  /?'s  field 
strengthened  a  little  to  prevent  reversal  in  case  the  speed, 
and  hence  voltage,  of  £'s  armature  may  chance  to  fall. 
The  load  can  now  be  gradually  transferred  from  A  to  B 
by  strengthening  B's  field,  and  at  the  same  time  weaken- 


308  TESTING    OF    DYNAMOS    AND    MOTORS. 

ing  A's.  It  may  happen  that  when  the  meter  is  placed 
across  the  switch,  instead  of  reading  nearly  zero  it  reads 
twice  the  line  voltage.  This  would  indicate  the  machine 
and  line  to  be  in  series  instead  of  multiple,  and  would 
necessitate  recharging.  Charging  may  be  avoided  by 
connecting  the  field  above  the  switch,  thus  exciting  from 
the  line  direct.  When  there  is  any  uncertainty  as  to 
connections  the  above  test  must  be  fully  made,  but  where 
there  is  absolute  certainty  of  correct  polarity  it  suffices 
to  use  a  test  lamp  instead  of  a  voltmeter,  or  even  a 
marked  position  on  the  field  rheostat.  However,  the 
position  of  the  rheostat  varies  considerably  for  different 
line  loads  and  different  temperature  of  the  machine,  so 
that  the  attendant  must  exercise  considerable  judgment. 
The  test  lamp  is  applied  in  the  same  way  as  the  volt- 
meter, and  when  it  burns  above  the  switch  with  the  same 
brilliancy  as  below,  the  switch  can  be  closed,  but  as  an 
extra  precaution  the  lamp  should  be  tried  across  the 
switch.  In  doing  this  care  must  be  had,  for  if  the 
machines  should  happen  to  be  in  series  the  lamp  will  be 
subjected  to  double  its  normal  voltage  and  will  explode, 
not  only  jeopardizing  the  operator's  eyes,  but  possibly 
injuring  the  armature  with  broken  glass.  To  lessen 
such  liabilities  it  is  well  to  use  two  lamps  in  series.  In 
station  practice  connections  are  permanent,  and  the  test 
across  the  switch  is  omitted.  Lines  are  run  from  all 
machines  to  the  switchboard,  as  are  also  a  pair  of  lines 
from  the  bus  bars:  by  means  of  a  key  the  voltmeter  can 
be  connected  alternately  to  the  bus  bars,  and  to  the 
terminals  of  the  dynamo  to  be  put  on.  When  the  volt- 
ages are  equal  the  switch  is  closed.  When  K'  is  closed 
B  does  not  begin  to  take  a  load  until  its  field  is 


SHUNT    AND    COMPOUND    MACHINES.  309 

strengthened,  and  for  this  reason  other  machines  in  ser- 
vice support  a  certain  E.  M.  F.  between  S  and  /•*,  to 
which  Z?'s  terminals  are  attached.  B  supports  the  same 
K.  M.  F.  between  S  and  P  in  opposition  to  that  already 
existing;  /'being  a  point  in  common  to  both  circuits, 
and  the  potential  on  both  sides  of  K'  being  the  same,  cur- 
rent does  not  flow  when  K'  is  closed.  Before  the  switch 
is  closed  there  is  below  it  the  total  E.  M.  F.  of  /?,  while 
above  is  the  terminal  potential  of  A\  A's  total  E.  M.  F. 
being  much  greater,  B  can  share  the  load  only  when  its 
total  E.  M.  F.  exceeds  the  terminal  E.  M.  F.  of  A. 
This  condition  is  secured  by  strengthening  /?'s  field  or 
weakening  A's.  When  there  are  many  machines  in 
multiple  the  latter  procedure  is  impracticable.  As  />'s 
field  is  strengthened  /?'s  E.  M.  F.  rises,  and  for  a  line 
current  of  given  value  the  line  E.  M.  F.  does  also,  because 
the  internal  resistance  of  two  or  more  machines  in 
multiple  is  less  than  that  of  a  single  one;  there  is,  there- 
fore, less  internal  drop,  and  the  E.  M.  F.  available  at  the 
terminals  common  to  all  the  machines  is  greater. 

After  adjusting  the  loads  on  all  the  machines  at  their 
proper  values,  it  is  well  to  keep  an  eye  on  the  newly  intro- 
duced machine,  until  it  has  become  throughly  heated, 
because  a$  its  armature  and  fields  rise  in  temperature 
their  resistance  increases,  with  the  result  that  the  internal 
drop  becomes  greater,  and  the  field  takes  less  current— 
both  effects  conspiring  to  lower  the  terminal  E.  M.  F.  and 
to  make  the  machine  shirk  its  load.  To  the  competent 
and  experienced  practical  man  such  simple  admonitions 
may  seem  uncalled  for,  but  to  the  plodding  beginner 
these  little  points  become  the  food  of  much  thought. 
Another  effect  of  heating  is  to  raise  the  reluctance  of 


310  TESTING    OF    DYNAMOS    AND    MOTORS. 

the  magnetic  circuit,  thereby  weakening  the  field  and 
further  lowering  the  E.  M.  F.,  and  when  we  remember 
that  each  machine's  load  depends  upon  its  E.  M.  F.,  it  is 
not  hard  to  realize  the  importance  of  these  influences. 
Should  any  part  of  the  magnetic  circuit  become  impaired, 
as  by  a  loose  or  rusty  joint,  the  effect  is  the  same  as  the 
above.  Such  a  defect  in  a  dynamo  cuts  down  its  E.  M.  F. 
and  reduces  its  load;  on  a  motor  it  cuts  down  the 
C.  E.  M.  F.  and  increases  the  load. 

The  effect  of  throwing  an  additional  machine  in  mul- 
tiple with  others  is  not  only  to  increase  the  current 
capacity  of  the  plant,  but  to  also  decrease  its  internal 
resistance.  The  lower  the  external  resistance  the  greater 
the  useful  effect  of  putting  on  an  additional  machine, 
because  if  the  resistance  of  feeders  and  mains  is  too 
great,  the  external  resistance  will  be  high  compared  to 
the  internal,  and  the  latter  will  be  but  a  small  part  of  the 
total,  and  lessening  this  small  part  has  no  apppreciable 
effect.  To  use  a  familiar  analogy,  take  the  case  of  a 
battery  working  through  an  external  resistance  of  1,000- 
ohms.  It  would  be  useless  to  put  any  number  of  cells  in 
multiple  on  such  a  line,  for  the  1,000  ohms  is  a  so  much 
greater  resistance  than  the  cell  part  of  the  circuit,  that 
any  decrease  in  the  latter  cannot  be  detected. 

The  problem  complementary  to  that  of  "  putting  on  " 
a  machine,  is  "  taking  off  "  one.  This  occurs  when  the 
station  load  falls  below  the  normal  capacity  of  the 
machines  in  service.  Of  course  one  way  to  take  off  a 
machine  is  to  pull  its  switch,  and  this  is  practiced  in 
emergencies,  but  ordinarily  is  open  to  several  objec- 
tions; in  the  first  place  on  machines  of  large  current 
capacity  and  of  sufficient  voltage  to  support  a  healthy 


SHUNT    AND    COMPOUND    MACHINES.  ?  I  I 

arc,  the  switch  blade  and  jaws  soon  blister  and  burn: 
secondly,  on  some  machines  using  carbon  brushes,  and 
on  all  machines  using  copper  ones  destructive  sparking 
attends  a  sudden  removal  of  load.  Thirdly,  where  there 
are  but  two  or  three  machines  working,  the  sudden 
removal  of  one  may  overload  the  rest  and  give  rise  to 
belt  troubles. 

Another  way  to  take  off  a  machine  is  to  shut  down  the 
engine  which  runs  it;  this  can  be  done  only  when  a 
machine  is  alone  on  the  circuit.  Under  no  circumstances 
must  the  steam  be  cut  off  from  an  engine  connected  to  a 
dynamo  running  in  multiple  with  other  dynamos,  driven 
from  a  different  engine;  for  the  steamless  engine  will 
continue  to  run  driven  by  its  dynamo,  which  has  become 
a  motor.  If  the  dynamos  are  simple  shunt  machines 
there  may  be  no  fireworks,  but  if  they  be  compound- 
wound  or  series  machines,  there  will  be.  The  following 
method  covers  all  cases  where  two  or  more  shunt 
dynamos  run  in  multiple:  where  practicable  strengthen 
the  fields  of  the  dynamos  that  are  to  remain  in  circuit 
and  weaken  the  field  of  the  one  to  be  removed,  keeping 
watch  over  the  machine's  ammeter  to  avoid  reversal; 
over  the  station  voltmeter  to  keep  up  the  voltage;  and 
over  the  brushes  to  shift  them  if  necessary.  When  the 
current  falls  nearly  to  zero  the  switch  must  be  opened, 
and  then  the  field  circuit  broken.  In  no  case  should  the 
field  be  broken  first.  Where  it  is  not  practicable  to 
change  the  rheostats  simultaneously  as  above,  the  line 
voltage  may  require  adjustment  afterward,  for  the  effect 
of  removing  a  machine  is  to  increase  the  internal  resist- 
ance and  the  internal  drop.  The  larger  the  number  of 
machines  in  multiple  the  less  the  disturbing  effect  of  any 


312  TESTING    OF    DYNAMOS    AND    MOTORS. 

one's  removal.  In  bringing  down  the  load  by  means  of 
the  field  rheostat  each  increase  of  resistance  should  be 
allowed  to  have  its  full  effect  before  introducing  any 
more,  for  magnetization  takes  an  appreciable  time  to 
respond  to  any  change  in  the  magnetizing  force.  Wher- 
ever any  load  or  speed  is  regulated  with  a  rheostat,  it 
saves  time  and  trouble  to  observe  this  precaution.  On 
the  same  principle  the  ammeter  needle  will  lag  behind 
the  current;  and  the  current  may  reach  the  point  of 
reversal  some  time  before  the  needle  indicates  zero,  so 
on  this  account  it  is  customary  to  open  the  switch  on  the 
safe  side  of  zero.  This  is  a  point  which  the  attendant 
should  understand,  for  if  the  machine's  E.  M.  F.  is 
allowed  to  fall  below  that  of  the  line,  reversal  takes 
place,  and  the  accompanying  sparking  may  injure  the 
commutator  as  much  as  several  months'  ordinary  wear. 
Aside  from  sparking  (which  may  be  very  light  under 
favorable  circumstances)  and  the  reversal  of  the  ammeter 
needle  (which  takes  place  only  on  the  Weston  type  of 
direct  current  meter)  there  are  other  reversal  signs  to  be 
looked  for :  (i)  motor  brushes  have  a  backward  lead,  if  any, 
under  load,  and  dynamo  brushes  a  forward  lead;  (2)  the 
tight  side  of  the  belt  always  runs  toward  the  driving 
pulley,  and  should*  be  on  the  bottom,  so  that  the  sag  of 
the  slack  side  shall  increase  the  pulley  contact.  If  then 
the  bottom  side  of  a  dynamo  belt  is  seen  .to  be  flapping 
or  sagging,  it  indicates  that  its  armature  is  pulling  on 
the  belt  and  is  therefore  a  motor.  Where  suspicion  is 
aroused  the  real  test  lies  in  strengthening  the  field  and 
observing  the  ammeter  needle;  if  the  machine  is  a  motor 
the  load  will  decrease  as  the  field  is  strengthened.  By 
looking  at  Fig.  108  it  can  be  seen  that  when  two 


SHUNT    AND    COMPOUND    MACHINES. 


3*3 


dynamos  run  in  multiple  the  E.  M.  Fs.  of  the  two  arma- 
tures oppose  each  other,  but  concur  in  direction  in  the 

outside  circuit;  if  the  line 
switch  be  opened,  as  at 
K,  there  is  no  common 
external  path,  and  the  op- 
position of  the  two  E.  M. 
Fs.  can  be  represented 
by  the  two  arrow  heads 
pointed  against  each  other 
in  circle  A  B  C  of  Fig. 
109.  If  K  be  closed,  and 
there  is  not  too  great  a  difference  between  the  voltages 
of  the  two  sources,  they  will  take  the  common  path  D  L 
C,  If,  however,  the  E.  M.  F.  of  one,  B  say,  is  much 
greater  than  that  of  the  other,  it  sends  backward  through 
A  against  its  weaker  E.  M.  F.  a  current  which  runs  it 
as  a  motor.  This  takes  place  when  ^'s  terminal  E.  M.  F. 
is  greater  than  A's  total 
E.  M.  F.  The  violence  of 
the  demonstration  which 
follows  a  shunt  machine's 
reversal  depends  upon  the 
discrepancy  between  the 
two  E.  M.  Fs.  If  this  is 
very  little,  it  may  not  be 
noticeable  that  one  machine 
is  running,  as  a  motor;  on  FIG.  109. 

the  other  hand   if   the  field 

be  broken  on  one  machine  its  armature  will  have  no 
opposing  power  at  all,  and  a  short  circuit  follows,  with 
tremendous  sparking. 


314  TESTING    OF    DYNAMOS    AND    MOTORS. 

We  have  learned  that  the  terminal  voltage  of  a  shunt 
machine  falls  as  the  load  increases,  but  that  up  to  a  certain 
limit  that  of  a  series  machine  rises.  It  was  early  sug- 
gested that  such  a  combination  of  these  two  machines 
might  be  made  as  would  give  between  certain  limits  a 
constant  terminal  potential.  The  result  of  this  sugges- 
tion is  embodied  in  the  compound-wound  dynamo. 
Compounding  consists,  as  the  name  indicates,  in  placing 
both  shunt  and  series  coils  upon  the  field  cores,  and  in  so 
proportioning  their  magnetizing  effects  that  any  tendency 
of  the  shunt  field  to  weaken  is  met  by  a  counter  tendency 
of  the  series  field  to  strengthen.  Stated  thus  boldly  the 
problem  may  appear  to  be  simple;  but  there  are  modify- 
ing factors  which  render  it  more  complicated  than  it  at 
first  appears.  These  we  will  take  up  in  order.  In  the 
first  place,  if  a  dynamo  has  been  compounded  cold  it  will 
be  under-compounded  when  hot;  and  if  compounded  hot 
will  be  over-compounded  when  first  started  up.  This  is 
due  to  the  fact  that  as  the  temperature  varies  so  does  the 
resistance  of  field  and  armature,  and  also  the  reluctance 
of  the  magnetic  circuit.  Since,  however,  all  machines 
after  running  several  hours  reach  a  stable  condition  as 
regards  temperature,  these  effects  may  be  considered  as 
fixed  quantities  to  be  allowed  for.  If  a  dynamo  has  been 
successfully  compounded  its  terminal  potential  remains 
practically  constant  for  all  loads,  and  the  shunt  winding 
will  be  subjected  to  a  constant  voltage;  the  shunt  field 
ampere-turns  will  then  be  constant  after  the  temperature 
becomes  so;  hence  in  calculations  for  compounding,  the 
shunt  field  is  considered  constant,  and  has  such  a  value 
as  will  give  the  required  voltage  on  open  circuit  when 
the  series  coils  are  inactive. 


SHUNT    AND    COMPOUND    MACHINES.  315 

The  third  modifying  factor  is  the  /  J?  drop  through 
the  armature  and  series  winding.  The  drop,  being  due 
to  resistance,  would  vary  with  temperature,  so  the  resist- 
ances are  always  taken  at  maximum  temperature  and 
considered  constant.  If  R  is  a  constant,  then  the  drop 
or  loss  of  potential  is  directly  proportional  to  /;  /'.  e., 
to  the  load.  If  the  drop  at  quarter  load  is  10  volts,  then 
at  full  load  it  will  be  40  volts.  To  compensate  for  this  loss 
it  is  convenient  to  know  the  loss  of  voltage  per  ampere 
of  current  in  the  armature.  Thus,  suppose  that  the  shunt 
field  on  open  circuit  gives  500  volts,  and  has  200,000 
ampere-turns.  This  gives  200,000  -f-  500  =  400  ampere- 
turns  per  volt.  Next  suppose  that  for  every  10  amperes 
of  armature  current  the  /  R  loss  is  i  volt;  this  means 
i/ioof  a  volt  per  ampere,  or  40  ampere-turns  per  ampere. 
For  every  ampere  in  the -armature  there  must  be  in  the 
series  field  40  ampere-turns,  so  that  for  a  current  of  100 
amperes  the  series  field  must  provide  100  x  40  =  4,000 
ampere-turns,  which  is  just  sufficient. 

A  machine  compounded  as  above  would  still  be  defi- 
cient, for  there  is  a  fourth  factor  to  be  allowed  for.  In 
Chapter  I.,  on  the  "Elementary  Theory  of  Dynamos," 
mention  was  made  of  the  armature  " reaction,"  or  "back 
induction,"  and  the  term  back  ampere-turns  was  ex- 
plained. The  measure  of  this  armature  reaction  was 
found  to  be  the  product  of  the  armature  current  and  the 
number  of  armature  coils  included  between  the  double 
angle  of  lead.  On  well-designed  compound-wound 
machines  the  lead  is  the  same  for  all  loads,  so  that  the 
"  back  ampere  turns  "  are  proportional  to  the  load  and 
equal  to  Tb  /,  where  Tb  is  the  number  of  armature  coils 
included  in  the  double  angle  of  lead.  Call  this  10; 


316  TESTING    OF    DYNAMOS    AND    MOTORS. 

then  the  back  ampere-turns  per  ampere  is  10.  This, 
added  to  the  40  necessary  to  overcome  the  /  R  loss, 
gives  50  as  the  number  of  ampere-turns  to  be  furnished 
by  the  series  windings  for  each  ampere  of  armature  cur- 
rent.  At  full  load,  100  amperes,  the  series  winding 
would  give  100X50  =  5,000  Si,  and  5,000  -^  100  =  50,  the 
number  of  series  turns.  Under  these  conditions  regula- 
tion should  obtain  from  no  load  to  such  load  as  the  design 
calls  for.  That  there  is  a  maximum  limit  for  regulation 
naturally  suggests  itself,  and  this  limit  is  found  as  fol- 
lows: In  Chapter  I.  it  was  shown  that  iron,  under  the 
influence  of  a  magnetizing  force,  is  magnetized,  and  its 
magnetism  increases  as  the  magnetizing  force  increases, 
until  a  point  is  reached  where  the  magnetism  is  but 
slightly  affected,  even  when  the  magnetizing  force  is 
greatly  increased.  At  this  point  the  iron  is  said  to  be 
saturated.  Next,  we  note  that  in  a  compound-wound 
machine  the  object  sought  is  to  secure  a  constant  ter- 
minal potential,  and  that  this  is  effected  by  varying  the 
armature  E.  M.  F.  Since  the  armature's  speed  and 
number  of  conductors  are  constant,  raising  the  E.  M. 
F.  is  effected  by  strengthening  the  magnetic  field, 
and  the  regulation  is  perfect  only  so  long  as  the  iron's 
magnetism  increases  at  the  same  rate  as  the  magnetizing 
force.  For  a  while  this  condition  is  satisfied,  but  as  soon 
as  the  iron  approaches  the  saturation  point  the  condition 
is  departed  from  more  and  more,  till  finally,  even  by 
doubling  the  magnetizing  force  (the  series  ampere-turns), 
the  iron  responds  but  little.  From  this  point  on  the 
terminal  voltage  decreases  as  the  current  increases,  be- 
cause the  voltage  loss  due  to  increased  reaction  and 
increased  /  J?  drop  is  greater  than  that  gained  by  the 


SHUNT    AND    COMPOUND    MACHINES.  317 

extra  turns.  The  range  of  regulation  in  a  machine 
depends  upon  the  design  and  upon  the  grade  of  iron 
used  in  the  fields,  cast  iron  having  a  lower  saturation 
point  than  wrought  iron  or  steel.  Where  the  question  of 
weight  is  important,  the  cross-section  of  the  field  is  a 
minimum,  so  that  even  on  open  circuit,  when  the  shunt 
coils  alone  are  active,  the  magnetization  of  the  iron  is 
rather  high.  Under  these  circumstances  the  saturation 
point  is  soon  reached,  and  full  load  voltage  will  probably 
be  slightly  lower  than  that  at  one-half  or  three-fourths 
load.  If  weight  and  size  are  secondary  considerations, 
the  field  cores  can  be  made  of  greater  cross-section,  so  that 
on  open  circuit  the  requisite  field  is  secured  with  a  low 
magnetization  value  in  the  iron,  and  the  range  of  regula- 
tion is  increased.  If,  in  testing  a  number  of  similar 
machines,  one  gives  voltage  readings  below  the  average, 
particularly  at  full  load,  it  is  quite  possible  that  an  inferior 
quality  of  iron  has  found  its  way  into  the  armature  or 
field  core.  Before  drawing  such  a  conclusion,  however, 
all  other  sources  of  error  must  be  carefully  eliminated. 
All  magnetic  joints  should  be  inspected  and  found  tight. 
The  bore  of  the  pole  pieces  should  be  calipered  as  well 
as  the  diameter  of  the  armature  core.  The  resistance  of 
armature  and  both  field  windings  must  be  taken,  and, 
finally,  the  whole  test  should  be  run  over  with  a  new  set 
of  instruments  and  with  the  men  differently  disposed. 
This  will  eliminate  errors  of  observation  and  those  due 
to  faulty  instruments.  If  a  machine's  dimensions,  the 
quality  of  its  iron,  and  all  winding  data  are  given,  it  is 
possible  to  calculate  very  closely  the  regulation  range; 
but  all  commercial  machines  are  put  through  a  thorough 
test,  and  finally  adjusted  experimentally.  This  adjust- 
ment is  later  considered  in  detail. 


.318  TESTING    OF    DYNAMOS    AND    MOTORS. 

In  actual  practice  the  compounding  of  a  dynamo  is  not 
as  much  of  an  undertaking  as  it  might  seem,  and 
has  this  advantage  over  the  engineer's  calculations,  that 
the  data  is  derived  from  the  machine  itself,  and  not  from 
tests  on  previous  machines;  especially  is  this  an  advan- 
tage where  new  types  of  machine  are  being  perfected. 
Suppose  that  we  have  on  hand  a  shunt  dynamo,  which  it 
is  desired  to  compound — a  case  very  likely  to  arise  out 
on  the  road,  where,  for  instance,  it  is  necessary  to  adapt 
a  shunt  machine  to  respond  to  the  sudden  fluctuations  of 
a  street  railway  circuit.  There  are  two  methods  of  pro- 
cedure, both  of  which  are  equally  practical  and  practicable, 
and  give  satisfactory  results.  The  following  requires  no 
data  and  is  free  from  calculation:  There  should  be 
available  a  spool  of  stranded  copper  cable  of  about  the 
same  cross-section  as  one  of  the  brush-holder  cables, 
also  two  heavy  clamps  to  serve  as  temporary  terminals 
for  the  cable.  The  method  consists  in  the  actual  placing 
of  successive  turns  of  wire  on  the  two  field  spools  and 
•connecting  them  in  series  with  the  armature  and  external 
line.  The  machine's  open  circuit  voltage  is  adjusted  to 
nominal  value,  and  the  rheostat  thereafter  left  undis- 
turbed. The  load  is  now  gradually  increased  by  cutting 
in  more  lamps  or  otherwise  lessening  the  external  resist- 
ance, and  the  terminal  voltage  watched;  if  it  falls  off,  the 
series  turns  must  be  increased.  If  the  voltage  raises  as 
the  load  does,  the  machine  is  over-compounded,  and  some 
turns  must  be  taken  off.  This  process  of  hit  and  miss  is 
continued  until  a  satisfactory  combination  is  secured. 
To  make  room  for  the  added  coils  the  rope  covering  on 
each  spool  may  be  removed.  A  layer  of  series  winding 
is  temporarily  placed  on  each  spool  in  such  a  way  that 


SHUNT    AND    COMPOUND    MACHINES.  319 

the  current  must  flow  in  opposite  directions  around  the 
two  spools.  To  effect  this,  the  wire  on  one  can  be  wound 
from  right  to  left,  looked  at  from  above,  and  on  the 
other  from  left  to  right,  and  the  inside  ends  connected 
together.  Or,  if  both  spools  have  been  wound  alike,  the 
inside  end  of  one  and  the  outside  end  of  the  other  are 
connected  together.  The  two  coils,  as  a  whole,  must 
now  be  connected  in  circuit,  so  that  the  armature  current 
passes  around  shunt  and  series  coils,  on  any  spool,  in  the 
same  direction. 

Having  approximately  determined  the  required  number 
of  coils  a  more  careful  test  is  made.  With  the  shunt  field 
thoroughly  heated,  the  rheostat  is  again  adjusted  to 
normal  voltage  and  the  load  put  on;  if  properly  com- 
pounded the  open  circuit  voltage  will  be  maintained 
throughout  the  load  range.  As  a  preliminary  test  the 
temporary  series  coils  should  be  cut  out  and  the  full 
current  load  put  on,  with  the  rheostat  undisturbed  from 
the  position  of  normal  voltage.  A  fall  in  voltage  will  of 
course  result,  and  its  amount  should  be  noted.  Then 
the  series  coils  should  be  cut  in  and  full  current  again 
put  on.  If  the  voltage  is  less  than  before,  it  indicates 
the  series  and  shunt  coils  to  be  opposed;  this  will  be  the 
symptom  if  there  are  but  a  few  series  turns  on;  but  if  the 
series  coil  is  complete,  its  opposition  will  make  it  impossi- 
ble to  get  on  very  much  of  a  load.  In  any  such  case  the 
connection  of  the  series  coil  is  the  one  to  reverse — not 
that  of  the  shunt  coil;  reversing  the  shunt  winding 
deprives  the  machine  of  ability  to  generate.  If  the  read- 
ing is  but  little  affected,  being  almost  what  it  was  at  full 
load  with  shunt  winding  alone,  it  is  indicative  that  the 
two  series  spools  oppose  each  other,  and  that  the  fall  in 


320  TESTING    OF    DYNAMOS    AND    MOTORS. 

voltage  is  due  to  their  resistance.  The  last  condition  is 
where  the  voltage  is  nearly  that  of  open  circuit,  and  it 
indicates  that  compounding  is  progressing  and  only  needs 
exact  adjustment;  this  is  done  by  adding  or  removing 
one  turn  at  a  time  and  successively  taking  readings. 
After  the  adjustment  of  series  turns  is  made  the  machine 
is  allowed  to  run  on  full  load  long  enough  to  bring  up 
the  series  coil  to  its  maximum  temperature.  At  the  end 
of  the  run  the  voltage  will  be  found  to  have  lowered  on 
account  of  the  rise  in  series  field  resistance;  and  this  loss 
must  be  compensated  by  putting  on  more  turns.  The 
cable  must  be  as  large  as  the  winding  space  will  permit, 
for  it  is  important  to  keep  down  the  loss  in  the  series  field 
itself.  A  machine  compounded  in  this  way,  with  all  its 
magnetic  and  electrical  losses  present,  is  compounded 
under  working  conditions,  and  can  be  relied  on  to  hold 
its  own  in  the  future  against  all  losses,  whether  due  to 
armature  reaction,  ohmic  resistance,  or  any  other  cause. 
After  a  compound  is  secured,  it  is  considered  good  prac- 
tice to  add  enough  series  turns  to  raise  the  full  load  volt- 
age 10  %  above  the  open  circuit  value.  Thus,  if  the  open 
circuit  voltage  be  500  volts,  the  full  load  voltage  will  be 
550  volts.  The  object  of  this  over-compounding  is  to 
enable  the  machine  to  maintain  a  uniform  delivery  volt- 
age at  some  point  whose  distance  will  not  entail  an  1  R 
loss  of  over  50  volts  at  full  load.  By  knowing  the  resist- 
ance of  the  feed  wire  or  main  between  this  point  and  the 
dynamo,  the  drop  can  be  found  and  series  coils  wound 
on  accordingly.  If  the  series  field  is  stronger  than  is  re- 
quired, its  effect  is  weakened  to  any  desired  extent  by  plac- 
ing in  multiple  with  the  winding  a  shunt,  as  shown  in  Fig. 
no.  Shunted  dynamos  are  most  generally  used  in  street 


S~\  ^nnnnnnnr 


J     A     I/ 

A      /      _/~vw~> 


^-^  s 


/K  L 


SHUNT    AND    COMPOUND    MACHINES.  321 

railway  work,  where  feeders  of  different  lengths  require 
different  degrees  of  over-compounding.  A  convenient 
form  of  shunt,  and  one  much  used,  is  made  of  German- 
silver  tape.  Knowing  from  previous  tests  or  by  calcu- 
lation the  approximate  resistance  of  the  shunt,  several 
lengths  of  tape  are  cut,  clamped  together  in  multiple,  and 
connected  across  the  series 
field,  as  shown  in  the  figure 
at  6".  One  of  the  clamps  is 
so  arranged  that  the  strips 
can  be  easily  slipped  through, 
and  the  shunt  thus  length- 
ened or  shortened.  Having 
placed  the  shunt  upon  the 

machine  the  load  is  adjusted  and  the  voltage  meas- 
ured; if  too  high  the  shunt  is  shortened,  thus  lowering 
its  resistance  and  diminishing  the  current  in  the  series 
coils.  If  the  voltage  is  too  low  the  shunt  is  length- 
ened or,  if  necessary,  an  entire  strip  can  be  cut  out. 
There  are  several  combinations  of  strips  that  will  give 
the  proper  resistance,  but  one  must  be  selected  that  will 
carry  the  current  without  heating  too  much.  The  office 
of  the  shunt  is  to  adjust  the  current  in  the  series  field 
winding,  and  thereby  to  secure  the  proper  magnetizing 
effect.  Having  thus  completed  the  shunt,  it  is  soldered 
at  its  ends,  and  becomes  an  integral  part  of  the  machine. 
Sometimes,  when  a  whole  plant  is  installed,  the  final 
compounding  is  done  after  the  machines  are  in  place. 
One  point  in  favor  of  this  practice  is  that  the  machines 
are  compounded  for  exactly  the  speed  at  which  they  are 
to  run,  whether  this  be  above  or  below  the  rated  speed 
or  not.  This  is  important,  for  a  machine  will  regulate 


322  TESTING    OF    DYNAMOS    AND    MOTORS. 

perfectly  only  at  the  speed  at  which  it  is  compounded, 
because  its  voltage  is  due  not  only  to  the  magnetic  field 
but  to  the  speed,  so  that  any  change  in  speed  changes 
the  voltage.  Furthermore,  this  change  in  voltage,  due 
to  speed  change,  reacts  upon  the  shunt  field,  strengthen- 
ing or  weakening  it  according  as  the  voltage  has  been 
raised  or  lowered.  Under  these  circumstances  rheostat 
regulation  must  be  used  so  long  as  the  speed  is  other 
than  that  at  which  the  machine  was  compounded. 
Proper  speed  is,  then,  of  prime  importance,  and  com- 
pounding in  situ  has  a  decided  advantage. 

In  practice  machines  are  frequently  compounded  to 
maintain  constant  potential  at  a  distant  point  on  the  line. 
A  machine  thus  appointed  is  said  to  be  over-compounded 
a  certain  per  cent,  above  the  normal  voltage,  according 
to  the  loss  to  be  allowed  for.  If  adjusted  for  a  10  % 
loss,  and  intended  to  give  500  volts  on  open  circuit,  the 
full-load  terminal  voltage  must  be  555  volts,  for  the  con- 
dition is  that  500  shall  be  90  <f0  of  full-load  voltage,  and  500 
volts  is  90  f0  of  555  volts.  Again,  if  a  20  fc  loss  is  to  be 
allowed  for,  the  full-load  terminal  voltage  must  be  625. 
When  such  heavy  over-compounding  is  called  for  it  is 
customary  not  only  to  increase  the  series  field  strength, 
tout  to  raise  the  speed.  This  calls  for  a  new  adjustment 
of  the  field  rheostat.  For,  in  any  case,  the  normal  voltage 
is  500,  and  it  stands  to  reason  since  the  voltage  is  due 
to  speed  and  field,  and  since  500  volts  are  due  to  the 
cutting  of  a  certain  number  of  lines  of  force  per  second, 
that  if  the  speed  is  increased  the  lines  must  be  decreased 
in  order  that  the  number  cut  per  second  may  be  the 
same,  and  500  volts  obtain.  To  decrease  the  number  of 
lines  resistance  is  put  into  the  field  rheostat  to  cut  down 


SHUNT    AND    COMPOUND    MACHINES.  323. 

the  field  current.  In  contracts  where  the  conditions  are 
previously  known,  the  machines  are  provided  with  special 
windings.  Where  machines  leaving  the  shop  are  com- 
pounded fora  particular  speed,  the  shunt  should  be  such 
as  to  admit  of  ready  variation  in  resistance.  Some 
makers  use  German-silver  tape;  others  adopt  a  variable 
shunt  board,  by  means  of  which  a  compound  can  be 
effected  for  a  10  %  variation  in  speed.  When  it  is  im- 
practicable to  run  the  speed  within  this  limit,  the  machine 
may  be  recompounded  as  above,  either  by  using  a  shunt 
or  ^v  adding  more  series  coils,  according  as  the  speed  is 
too  nigh  or  too  low.  When  a  compound-wound  dynamo's 
speed  is  increased,  the  effect  upon  the  two  windings  is 
different.  The  I*R  loss  in  the  armature  and  series  field 
remains  the  same  for  given  current,  but  the  voltage  con- 
tributed by  the  series  winding  varies  with  the  speed. 
Assuming  /to  be  kept  the  same  and  the  speed  tb  be  in- 
creased, the  series  Si  (ampere-turns)  remain  the  same. 
Any  increase  of  voltage  due  to  cutting  of  lines  of  force 
furnished  by  the  series  windings  is  due  to  increase  of 
speed  alone.  \Vith  the  shunt  windings  the  magnetic 
effect  is  augmented  by  not  only  the  increase  of  speed 
directly,  but,  since  the  voltage  applied  to  the  shunt  field 
is  raised,  to  the  increase  in  field  current.  At  variable 
speed,  then,  the  series  field  current  remains  the  same, 
but  that  in  the  shunt  field  does  not.  Assuming  the  open- 
circuit  voltage  to  be  the  same,  however  the  speed  may 
vary,  the  machine  will  still  fail  to  compound  at  any  other 
than  its  proper  speed,  for  according  as  the  speed  is  high 
or  low  the  series  effect  will  be  too  much  or  too  little. 

Often    in     testing   rooms    it   is    found    impracticable, 
for   one    reason   or   another,    to    run   a    machine   at   its. 


324  TESTING    OF    DYNAMOS    AND    MOTORS. 

proper  speed.  A  somewhat  curious  and  not  very  accu- 
rate circumlocution  is  then  resorted  to.  It  is  called 
''compounding  volts  per  speed,"  or  "volts  per  revolu- 
tion," and  is  based  upon  the  assumption  that  any  change 
of  speed  will  provide  a  like  change  in  the  voltage.  Thus, 
it  is  assumed  that  if  a  machine  generates  500  volts  at 
1,200  revolutions  per  minute,  it  will  give  1,000  volts  at 
2,400  revolutions  per  minute,  and  250  volts  at  600.  This 
assumption  may  be  made  without  serious  error  if  it  is 
experimentally  proven  that  the  open-circuit  voltage  fol- 
lows this  law,  but  it  is  not  at  all  likely  that  it  ever  does 
so.  It  is  much  more  satisfactory  to  determine  just  what 
effect  a  given  speed  variation  has  on  the  voltage,  and  to 
base  corrections  upon  this,  for  we  have  just  seen  that 
speed  variation  of  voltage  on  a  compound-winding  ma- 
chine follows  no  such  simple  law.  Compounding  volts 
per  revolution  is  carried  out  as  follows:  Suppose  we  have 
a  roo-volt  dynamo  whose  proper  speed  is  1,000  revolutions 
per  minute,  but  upon  taking  its  speed  it  is  found  to  be 
running  at  only  900  revolutions  per  minute.  This  is  100 
revolutions,  or  10  %  too  low.  The  normal  voltage  at 
the  rated  speed  would  be  100,  but  since  the  speed  is  but 
90  </0  of  its  proper  value,  the  voltage  is  adjusted  to  90  <f, 
of  its  proper  value,  and  this  gives  90  volts.  The  as- 
sumption is  that  if  a  speed  of  900  gives  a  voltage  of  90,  a 
speed  of  1,000  would  give  a  voltage  of  100.  The  load  is 
then  put  on,  and  the  same  performance  gone  through 
with. 

The  foregoing  hit-or-miss  method  of  compounding  is 
satisfactory  when  one  has  a  reel  of  cable  from  which,  he 
is  at  liberty  to  use  what  is  needed,  and  has  the  privilege 
of  returning  the  rest.  Frequently,  however,  the  cable 


SHUNT    AND    COMPOUND    MACHINES.  325 

must  be  ordered  and  paid  for  in  advance,  and  it  is  de- 
sirable to  know  about  how  much  is  needed,  so  as  to  avoid 
needless  expense.  Under  these  conditions- the  following 
procedure  is  good  practice:  If  a  voltmeter  is  available, 
adjust  the  machine  to  normal  voltage,  with  line  switch 
open  and  armature  at  right  speed.  Should  no  reliable 
voltmeter  be  obtainable,  any  device  that  can  be  used  to 
read  voltage  may  be  calibrated  from  the  dynamo's  pilot 
lamp.  With  the  lamp  at  proper  brightness,  as  far  as  the 
eye  can  judge,  the  indicator  needle's  position  is  marked, 
the  indicator  being  across  the  terminals.  Once  this  sin- 
gle point  is  determined  we  can  always  tell  when  the 
voltage  is  right.  Any  indicator  that  will  give  a  readable 
deflection  will  serve  the  purpose,  and,  if  necessary,  it  can 
be  placed  in  series  with  a  high  resistance  to  reduce 
the  deflection.  If  no  indicator  can  be  had,  the  ability 
of  the  eye  to  judge  the  pilot  lamp's  brightness  must 
be  relied  upon.  This,  while  not  very  accurate,  gives 
fair  satisfaction.  Knowing  the  voltage  which  the 
machine  is  giving,  and  keeping  this  constant,  we  know 
the  field  current  to  be  constant,  and  knowing  or 
measuring  the  resistance  of  the  field,  we  can,  from 
Ohm's  law,  get  the  value  of  this  current.  If  there  is  no 
way  of  measuring  the  field  resistance,  it  can  be  deter- 
mined approximately,  as  follows :  Remove  enough  of 
the  rope,  etc.,  to  enable  the  outside  diameter  of  the 
winding  to  be  measured;  remove  the  keeper  and  expose 
the  iron  field  core;  the  diameter  of  the  core,  less  1/4  inch 
to  be  allowed  for  insulation,  gives  the  inside  diameter  of 
the  winding;  this  taken  from  the  outside  diameter  gives 
the  radial  depth  of  the  winding.  The  radial  depth  divided 
by  the  diameter  of  the  wire  gives  the  number  of  layers; 


326  TESTING    OF    DYNAMOS    AND    MOTORS. 

the  length  of  the  core  divided  by  the  wire's  diameter 
gives  the  number  of  wires  per  layer;  the  number  per 
layer  multiplied  by  the  number  of  layers  gives  the  number 
of  turns  on  the  spool.  From  the  outside  and  inside  diam- 
ters  can  be  figured  the  average  length  of  a  turn,  from  the 
formula 


X  =    : 

2 

or, 

D  -f-  </ 

#  =  =Q-    X    7T, 

Where  Z>,  ^,  and  x  are,  respectively,  the  outside  and 
inside  diameters  and  the  average  length  of  a  turn.  The 
average  length  per  turn  multiplied  by  the  number  of 
turns  gives  the  length  of  wire  on  the  spool.  Taking  the 
diameter  of  the  wire,  not  including  the  insulation,  and 
consulting  a  resistance  table,  we  find  the  resistance  per 
foot;  this  multiplied  by  the  number  of  feet  equals  the 
resistance  of  the  wire  on  one  spool,  and  this  multiplied 
by  the  number  of  spools  equals  the  cold  resistance  of  field 
coils.  Next  subject  the  field  to  its  designed  voltage  till 
it  reaches  its  maximum  temperature.  From  the  rise  in 
temperature  get  the  rise  in  resistance,  and  we  have  the 
field  resistance  hot;  whence  the  field  current 

E 


where  R-^  is  the  field  rheostat  resistance.     As  a  last  re- 
sort, R^  can  be  gotten  by  substituting  for  k  a  wire  of 


SHUNT    AND    COMPOUND    MACHINES.  327 

known  dimensions  whose  resistance  is  readily  obtained 
either  by  calculation  or  from  a  table.  In  lieu  of  wire,  a 
combination  of  lamps  can  be  used.  Such  a  combination 
must  be  selected  as  will  either  bring  the  lamps  to  incan- 
descence or  not  heat  them  at  all,  for  there  is  a  great  dif- 
ference between  the  hot  and  cold  resistance  of  a  filament. 
Knowing  the  resistance  of  one  lamp  under  either  of  the 
above  conditions,  and  assuming  them  to  be  all  alike 
(rather  a  bold  assumption),  the  total  resistance  can  be 
roughly  approximated.  We  have  nov,  the  field  current 
and  the  number  of  turns  on  the  field,  from  which,  by 
multiplying,  we  get  the  field  ampere-turns  at  no  load  and 
proper  speed.  Taking  care  that  the  speed  does  not 
change,  full  load  is  now  put  on,  and  /the  voltage  kept  up 
by  the  rheostat.  In  varying  the  rheostat  to  meet  the* 
requirements  of  the  load  the  ampere-turns  of  excitation 
have  been  increased,  and  the  amount  of  this  increase  is 
an  exact  measure  of  the  Si  to  be  furnished  by  the  series 
windings,  and  must  be  measured.  To  do  this,  the  new 
value  of  the  field  circuit  resistance  must  be  determined. 
Since  the  field  resistance  remains  the  same,  the  problem 
is  to  again  find  what  j?b  is.  This  known, 


From  this  we  get  the  Si'  at  full  load,  and  the  difference 
between  Si  and  Si'  gives  the  ampere-turns  to  be  fur- 
nished by  the  series  coils.  To  find  the  number  of  series 
coils  needed  it  is  only  necessary  tc  divide  the  series  Si 
by  the  full  load  armature  current.  Thus,  suppose  the  no 
load  Si  to  be  25,000,  and  the  full  load  Si  to  be  28,000, 


328  TESTING    OF    DYNAMOS    AND    MOTORS. 

also  the  full-load   armature  current  to  be   100  amperes, 
then  the  number  of  series  turns  is 

28,000  —   2C.OOO 


or  15  turns  on  each  spool,  if  there  are  two  spools.  Know- 
ing the  amount  of  space  available  for  winding,  the  amount 
of  cable  necessary  is  readily  figured  and  ordered  accord- 
ingly, with  a  little  margin  for  terminals  and  final  experi- 
mental adjustment. 

The  above  method  is  incomplete  in  that  it  makes  no 
allowance  for  the  drop  through  the  series  coils  themselves. 
The  first  result  is  an  approximation  then,  and  must  be  cor- 
rected by  the  addition  of  sufficient  turns  to  care  for  the 
series  field  IR  loss  as  figured  from  the  size  and  length  of 
the  cable  used.  Each  addition  of  series  turns  further 
increases  the  resistance,  so  that  more  turns  must  be  added 
to  compensate  for  this  increase.  The  process  of  adding 
and  reading  can  be  carried  to  the  closest  approximation. 
The  method  may  seem  rather  tentative,  but  in  the  par- 
ticular case  it  is  assumed  that  no  data  is  held  and  that 
there  are  no  facilities  for  testing.  Should  it  develop  that 
the  iron  is  so  saturated  by  the  shunt  coils  that  series  coils 
have  too.  little  effect,  a  compound  can  be  effected  only  by 
running  the  dynamo  at  a  lower  voltage.  This  lessens  the 
shunt  field  lines  of  force  and  makes  more  room  for  the 
series. 

In  calculating  the  size  of  cable  for  use  as  series 
coils,  the  IR  loss  in  them  should  not  exceed  2  $  of  the 
terminal  voltage.  Knowing  the  length  of  cable  to  be 
put  on,  and  the  maximum  allowable  resistance,  a  resist- 
ance table  will  give  the  minimum  cross-section.  It  is 


SHUNT    AND    COMPOUND    MACHINES.  329 

well  to  keep  within  the  limit,  so  as  to  allow  for  any  neces- 
sary additional  turns.  Besides  the  desirability  of  minimiz- 
ing the  series  field  loss,  it  may  be  observed  that  if  too 
small  a  wire  is  used  it  will  be  impossible  to  compound  the 
dynamo  at  all.  This  condition  obtains  when  the  resist- 
ance of  each  added  turn  causes  an  additional  drop  greater 
than  the  increase  of  voltage  which  its  magnetizing  effect 
produces.  Each  turn  must  therefore  supply  more  voltage 
than  it  itself  consumes. 

Series  windings  on  small  dynamos  are  generally 
stranded  copper  wire,  thus  offering  more  radiating  sur- 
face for  given  sectional  area  than  solid  wire  does.  On 
large  machines  flat  copper  tape  is  used,  being  almost  as 
flexible  and  more  compact  than  the  stranded  cable,  and 
giving  a  larger  radiating  surface.  Where  an  old  shunt 
machine  is  compounded,  the  series  turns  are  wound 
outside  the  shunt,  but  on  new  machines  the  series 
coils  are  placed  underneath:  the  tape  or  strip  copper 
being  lathe  wound  between  alternate  layers  of  insulation. 
The  reason  old  machines  take  the  series  coils  outside  is 
that  the  shunt  coils'  presence  makes  it  cheaper  and 
more  convenient  to  do  so:  new  machines  take  them 
inside  because  that  in  order  to  keep  the  IR  drop  low  the 
cross-section  of  the  series  conductor  is  comparatively 
•  great,  and  the  current  density,  hence  the  heating,  is  less 
than  in  the  shunt  field,  which  is  therefore  put  outside 
next  to  the  air.  Aside  from  this,  a  fine  wire  winding  tends 
to  heat  more,  because  the  wires  lie  closer  together  and 
lessen  the  intermediate  air  spaces.  When  not  wound  one 
over  the  other,  the  shunt  and  series  coils  are  wound  side 
by  side,  each  having  full  radial  depth.  This  enables 
either  one  to  be  removed  without  disturbing  the  other. 


330  TESTING    OF    DYNAMOS    AND    MOTORS. 

It  simplifies  connecting  to  wind  shunt  and  series  coils  in 
the  same  direction  on  the  spool.  Before  making  perma- 
nent connections,  temporary  ones  can  be  used  for  getting 
the  two  windings  of  the  same  and  alternate  pole  pieces 
of  opposite  polarity.  In  using  the  pilot  lamp  as  a  guide, 
allowance  must  be  made  for  the  fact  that  as  a  rule  it 
burns  brighter  than  the  lamps  out  on  the  line. 


CHAPTER  X. 

THE    COMPOUND-WOUND    DYNAMO. — GENERAL    TESTS. 

A  COMPOUND-WOUND  machine,  partaking  as  it  does  of 
the  nature  of  a  shunt  and  series  machine,  possesses  the 
characteristics  of  both;  like  a  shunt  dynamo  it  maintains 
its  voltage  on  open  circuit,  and  like  a  series  machine  it 
maintains  its  field  on  short  circuit,  and  will  give  trouble 
with  belts  or  clutches  unless  the  cut-outs  act  promptly. 
As  with  shunt  machines,  the  independence  of  its  shunt 
field  adapts  it  for  running  in  multiple  with  other  ma- 
chines; but  the  instability  of  its  series  field  requires  the 
UFC  of  a  special  regulating  device  called  an  "equalizer." 

The  office  and  action  of  an  "  equalizing  bar"  is  un- 
derstood by  comparatively  few.  Its  object  is  to  enable 
each  of  several  compound-wound  or  series  machines,  run- 
ning in  multiple,  to  take  its  share  of  the  load,  and  to 
make  them  independent  of  small  speed  variations.  Its 
action  is  as  follows:  Suppose  two  compound-wound 
machines  are  connected  in  multiple,  and  that  the  series 
and  shunt  coils  on  both  dynamos  are  cumulatively 
connected:  one  terminal  of  each  dynamo  goes  to  the 
negative  "bus-bar,"  as  shown  in  Fig.  in;  the  other 
terminal,  one  end  of  the  series  winding,  goes  to  the  pos- 
itive "bus-bar."  The  equalizer  runs  from  the  brush 
to  which  the  series  field  on  one  machine  is  attached  to 
the  corresponding  brush  on  the  other  machine.  It 

33« 


332 


TESTING    OF    DYNAMOS    AND    MOTORS. 


always  joins  the  two  brushes  which  are  at  the  same  po- 
tential, when  the  machines  run  at  the  same  voltage. 

If  by  mistake  the  equalizer  is  run  from  the  positive 
brush  of  one  dynamo  to  the  negative  brush  of  another, 
a  short  circuit  results  when  the  switches  are  closed,  be- 
cause (Fig.  in)  it  brings  together  the  positive  and 
negative  sides  of  the  same  dynamo.  It  is  necessary, 
therefore,  that  the  machines  should  be  of  such  polarity 


l^lVWWW — 
K'     i-V 


K 


FIG.   in. 


as  to  make  of  the  same  sign  all  brushes  attached  to  the 
series  fields.  This  condition  can  be  obtained  as  follows: 
a  voltmeter  is  placed  across  the  brushes  of  one  machine 
in  such  a  way  that  when  the  meter  circuit  is  closed  the 
needle  deflects  in  the  proper  direction;  the  line  con- 
nected to  the  brush  next  to  the  series  field  is  marked; 
placing  the  meter  across  another  machine,  with  the 
marked  line  on  the  series  field  brushj  the  deflection 


THE    COMPOUND-WOUND    DYNAMO.  333 

should  be  in  the  same  direction  as  before;  if  it  is  not, 
the  polarity  of  one  machine  must  be  reversed  before 
closing  the  line  switch.  This  can  be  done  by  shifting 
the  series  field  connection  to  the  other  side  of  the 
armature,  in  which  case  the  shunt  field  must  be  con- 
nected in  "long  shunt,"  otherwise  the  shunt  connections 
are  reversed  and  the  machine  will  refuse  to  generate;  it 
is  customary,  however,  to  effect  reversal  by  recharging 
the  shunt  field  from  the  line.  This  done,  and  the  volt- 
ages of  machine  and  line  equal  but  opposite,  the  meter 
should  read  zero  between  the  series  field  brushes,  and  it 
is  here  that  the  equalizer  is  connected.  If  one  machine 
runs  with  load  and  the  other  without  load,  the  brush  to 
brush  voltage  of  the  former  will  be  higher  than  that  of 
the  latter,  although  their  terminal  voltages  may  be  the 
same.  The  terminal  voltages  must  be  nearly  the  same, 
because  the  machines'  terminals  are  points  in  common 
on  the  bus-bars,  except  in  so  far  as  they  may  be  connected 
to  the  bars  at  some  little  distance  apart,  and  the  terminal 
voltage  of  two  machines  wiH  differ  only  by  an  amount 
equal  to  the  IR  drop  in  that  part  of  the  bus-bar  included 
between  them.  In  other  words,  the  machine  or  ma- 
chines already  in  service  practically  dictate  the  terminal 
voltage.  Their  brush  voltages  differ,  because:  every 
dynamo  under  load  has  an  external  and  an  internal  cir- 
cuit; if  there  are  no  motors  in  service  the  external  cir- 
cuit has  only  ohmic  resistance;  but  the  internal  circuit 
has  both  ohmic  and  inductive  resistance — ohmic  resist- 
ance due  to  the  armature  wire,  and  inductive  resistance 
due  to  the  very  force  which  produces  the  difference  of 
potential.  The  two  brushes  are  then  the  boundary 
line  between  the  external  and  internal  circuit,  and  as  far 


334  TESTING    OF    DYNAMOS    AND    MOTORS. 

as  concerns  one  machine,  its  series  field  is  part  of  its 
external  circuit.  If  a  voltmeter  be  placed  across  a 
machine's  terminals,  it  reads  the  drop  through  the 
external  circuit  less  the  series  field  1R  loss;  but  if  the 
meter  is  placed  across  the  brushes  it  reads  the  total 
external  drop,  which  is  therefore  greater  than  the  termi- 
nal voltage.  Now,  on  a  machine  which  is  up  to  voltage, 
but  is  taking  no  load,  no  current  flows;  there  is  no 
IR  loss,  hence  its  brush  and  terminal  voltage  are  the 
same.  The  machine  is  statically  charged  to  the  line's 
difference  of  potential,  and  the  entire  drop  takes  place 
across  the  generating  force  within  the  armature.  When 
one  machine  is  loaded,  then,  and  the  other  is  free,  the 
series  field  brushes  on  the  two  are  at  different  potentials, 
hence  so  are  the  two  ends  of  the  equalizer,  and  when  its 
switch  is  closed  current  will  flow  through  it  to  the  field 
of  the  free  machine:  the  equalizer  places  the  series  fields 
of  the  two  machines  in  multiple,  and  the  current  through 
each  will  depend  upon  their  relative  resistance  and  upon 
that  of  the  equalizer,  whose,  resistance  is  virtually  part 
of  the  series  field  resistance  of  the  idle  machine.  Hence 
the  importance  of  having  the  equalizer  short  and  stout. 

In  closing  an  equalizer  switch  there  are  produced  two 
effects:  The  loaded  machine's  series  field  is  weakened, 
for  part  of  its  current  flows  through  the  equalizer;  the 
free  machine's  series  field  is  strengthened;  hence,  if  the 
line  switch  is  closed  on  the  free  machine  just  after 
the  equalizer  switch,  the  free  machine  will  take  load,  and 
the  loaded  machine  lose  some.  The  process  of  equaliza- 
tion continues  until  the  brush  to  brush  potential  of  both 
machines  is  the  same,  when  no  current  flows  in  the  equal- 
izer. If  from  speed  variation  the  voltage  of  one  machine 


THE    COMPOUND-WOUND    DYNAMO.  335 

falls,  the  potential  of  that  end  of  the  equalizer  falls,  and  a 
current  flows  through  the  equalizer  to  the  series  field  of 
the  lower  potential  machine.  The  field  of  this  machine 
being  thus  strengthened,  its  voltage  rises  and  it  takes 
more  load.  Equalization  takes  place  promptly,  and  cur- 
rent flows  through  the  equalizer  first  one  way  and  then 
the  other,  and  sometimes  not  at  all. 

Any  attempt  to  run  compound-wound  or  series  machines 
in  multiple  without  either  rigid  connection  between  the 
armature  shafts  or  an  equalizer,  will  always  give  trouble. 
If  the  speed,  and  with  it  the  voltage,  of  one  machine  falls 
off,  the  other  machine  takes  the  extra  load,  and  in  this 
way,  strengthening  its  own  series  field,  induces  it  to 
further  overload  itself  and  unload  its  companion.  On 
machines  overcompounded  10  %  or  20  #,  the  trouble  is 
aggravated. 

There  is  one  condition  under  which  two  compound- 
wound  or  series  machines  will  run  together  in  multiple 
without  either  rigid  connection  or  equalizer,  and  this  is 
when  both  belts  are  free  to  slip;  it  is  then  impossible  to 
overload  either  machine,  for  as  soon  as  its  load  reaches  a 
certain  value,  the  belt  slips,  the  armature  slows  down,  the 
voltage  falls,  and  the  load  shifts  to  the  other  machine, 
which  in  turn  performs  the  same  cycle.  This  is  not  an 
accepted  mode  of  regulation,  for  aside  from  its  inefficiency, 
it  consumes  brushes  by  the  sparking.  Where  there  are 
more  than  two  compound-wound  machines  running  in 
multiple,  the  equalizing  bar  is  carried  from  machine  to 
machine  till  all  are  joined  together.  Each  machine  thus 
depends  for  regulation  upon  all  the  rest. 

To  introduce  a  compound-wound  machine  into  circuit 
with  other  compound. wound  machines  running  in  multi- 


336  TESTING    OF    DYNAMOS    AND    MOTORS. 

pie,  proceed  as  follows:  The  shunt  field  is  first  charged 
from  the  line,  and  the  open-circuit  voltage  adjusted  to 
the  proper  value,  and  opposed  to  the  line  voltage;  next, 
close  the  equalizing  switch,  thus  putting  a  series  field  on 
the  machine.  The  immediate  effect  is  to  raise  the 
machine's  voltage,  and  to  lower  that  on  the  line  by  an 
amount  depending  upon  how  many  machines  are  in  service. 
The  line  switch  is  next  closed,  and  the  machine  immedi- 
ately takes  its  load.  The  line  switch  is  not  closed  first, 
because  in  stations  wh^re  the  machines  are  heavily  over- 
compounded  the  terminal  voltage  of  the  loaded  machines 
exceeds  the  open  circuit  voltage  of  the  machine  to  be  put 
on,  and  closing  the  line  switch  first  under  these  condi- 
tions would  result  in  running  the  machine  as  a  motor, 
and  probably  in  the  wrong  direction.  With  the  equalizing 
switch  in  first  this  cannot  occur  for  the  reasons  stated 
above.  In  practice  a  three-pole  switch  is  arranged  to 
throw  in  the  equalizer  a  little  ahead  of  the  line  switch. 
With  the  shunt  field  across  the  line  it  is  impossible 
to  permanently  reverse  a  compound-wound  machine's 
polarity,  unless  the  polarity  of  all  becomes  reversed,  for, 
if  the  shunt  field  is  permanently  charged  from  the  line, 
its  polarity  is  fixed  so  long  as  the  connections  are  undis- 
turbed. In  all  cases  where  changes  or  repairs  have  been 
made  in  field  or  rheostat  wiring,  the  machine's  polarity 
should  be  tested.  The  equalizer  connections  must  be 
good,  and  the  bar  of  good  cross-section,  otherwise  the 
equalizer  fails  in  its  object  to  place  all  series  field  brushes 
at  the  same  potential.  Bad  regulation  between  com- 
pound-wound machines  has  frequently  been  traced  to 
insufficient  copper,  or  to  a  loose  connection  in  the 
equalizer. 


THE    COMPOUND-WOUND    DYNAMO. 


337 


The  discussion  so  far  has  involved  only  machines  of 
the  same  current  capacity  and  of  the  same  general  type. 
Often,  however,  it  is  desired  to  run  in  multiple  machines 
of  equal  voltage  but  different  load  capacities.  The 
equalizer  makes  this  possible,  provided  a  single  condition 
is  satisfied,  namely:  that  the  resistance  of  the  series 


FIG.  112. 

fields  be  made  inversely  proportional  to  the  intended  out- 
put of  the  machines,  so  that  for  any  variation  of  line 
resistance  the  variation  of  series  field  /  R  drop  shall  be 
the  same.  That  is  to  say,  if  A's  output  is  to  be  twice 
^'s,  £'s  series  field  resistance  must  be  twice  ^'s;  other- 
wise they  will  not  share  the  load  properly.  This  feature 
of  the  compound-wound  machine  is  a  series  machine 
characteristic,  and  is  best  understood  when  considered  in 
connection  with  them.  Fig.  112  gives  connections  for 


338  TESTING    OF    DYNAMOS    AND    MOTORS. 

two  series  dynamos  running  in  multiple,  with  an  equalizer 
between  them.  A  and  B  are  the  armatures  of  the  two 
machines,  and  Sl  and  S9  their  series  fields  respectively;  K 
is  the  equalizer  switch  and  K ',  K"  those  of  B  and  A. 
A  is  the  machine  of  larger  output,  say  twice  that  of  JB, 
and  the  resistances  of  Sl  and  S^  we  suppose,  for  illus- 
tration, to  be  the  same.  Neglecting  the  resistance  of  the 
equalizer,  the  effect  of  closing  K  is  to  place  in  multiple 
two  fields  (Slt  S9)  of  equal  resistance;  these  fields  will 
therefore  take  equal  currents  if  one  machine  is  running 
with  load  at  the  time  K  is  closed.  This  current  in  itself 
would  perhaps  be  sufficient  to  injure  the  series  field  of 
the  smaller  machine  even  were  its  line  switch  never  closed, 
so  that  the  armature  could  do  work.  Aside  from  this,  if 
the  two  fields  have  the  same  number  of  turns  the  smaller 
machine's  field  will  be  disproportionately  strong,  and  if 
smaller  machine  has  more  turns,  as  it  is  likely  to  have, 
this  disproportion  increases,  so  that  for  all  loads  the 
smaller  machine  takes  more  than  its  share,  while  the 
larger  machine  takes  less,  and  the  equalizer  is  never  idle. 
To  keep  the  load  proportionately  shared  either  of  two 
things  must  be  done:  First,  The  rheostat  of  the  smaller 
machine  must  be  constantly  changed  to  suit  variations  of 
load  just  as  on  a*  shunt  machine;  on  circuits  subjected  to 
sudden  and  violent  changes,  such  as  are  found  on  street 
railway  circuits,  this  is  impracticable,  and  where  it  is 
tried  sparking  and  belt  slipping  ensues;  in  any  case,  we 
lose  the  most  valuable  property  of  a  compound-wound 
machine,  namely,  its  regulative  power,  for  a  compound- 
wound  machine  will  regulate  for  constant  potential  only 
when  its  rheostat  is  adjusted  for  normal  voltage  on  open 
circuit,  and  left  in  that  position,  the  series  winding  pro- 


THE    COMPOUND-WOUND    DYNAMO.  339 

viding  the  additional  voltage  necessary  to  care  for  load 
losses.  Second,  The  series  field  of  the  smaller  machine 
must  be  weakened,  and  this  is  done  by  putting  in  series 
with  it  a  resistance  (such  a  resistance  must  be  inserted 
beyond  the  point  where  the  equalizer  taps  on,  so  as  not 
to  be  in  circuit  when  current  flows  from  B  to  A}.  Sup- 
pose A  has  a  current  output  of'  300  amperes,  and  B,  of 
100  amperes,  and  that  A's  series  field  resistance  is  .002 
ohm:  What  should  that  of  7?'s  be?  Since  /?'s  current  is 
one-third  of  A's,  7?'s  field  resistance  must  be  three 
times  A's;  or  3  x  .002  =  .006  ohm.  If  it  happens  to 
be  only  .0042  ohm,  we  must  add  an  extra  resistance  of 
.006  --  .0042  =  .0018  ohm,  which  had  preferably  be 
put  between  the  series  coils  and  the  positive  bus-bar. 
On  the  other  hand,  if  77' s  field  resistance  is  too 
great,  or  A's  too  small,  the  extra  resistance  must  be 
placed  in  A's  field.  In  general,  calling  7A,  7B  the  respec- 
tive currents,  and  J?S1  ^'8the  respective  resistances  of  the 
fields  of  A  and  B,  then  must  R^  :  R\  \  \  7B  :  7A,  or  in 
words:  to  have  proper  regulation  the  series  field  resist- 
ances must  be  inversely  as  the  maximum  currents  they 
are  to  carry.  Multiplying  means  together  and  extremes 
together,  we  have  fis  7A  =  R'g  IB,  whence 

J?8  =  £-  *'.,  and  R\  =  'A-  R%  . 

yA  JV 

Take  the  example  analyzed  above:  7A  =  300,  7B  =  100; 
J?s  =  .002;  jR'n  —  ?  From  the  above 

•R'*=  -T-  <RS  =  X  .002  =  .3  x  .002  =  .006  ohm. 

JB  IOO 

Suppose  .#'s  field  resistance  =  .008  ohm:  What  must 
A's  be?  7A  =  300,  7B  =  100;  £'R  =.008:  ^8  =  ? 


340  TESTING    OF    DYNAMOS    AND    MOTORS. 

J?s  =  -p  X  ^'s  = X  .008  =  : =  .00266  ohm. 

A  30°  3 

Since  A's  measures  .002,  an  extra  resistance  —  .00266  — 
.002  =  .00066  ohm  must  be  added  to  A's  field.  This  extra 
resistance  consumes  a  small  amount  of  power  which  is 
the  price  paid  for  good  regulation.  In  the  cases  above, 
suppose  the  machines' 'voltage  to  be  500.  Then  the 
watts  output  of  each  will  be: 

A  =  500  x  300  =  150,000  watts 
JS  =  500  x  ioo  =     50,000  watts. 

Watts  wasted  in  A's  resistance  =  .00066  X  3oo2  = 
.00066  x  90,000  =  59.4  watts,  or  about  what  is  con- 
sumed in  a  16  candle-power  lamp. 

Compound-wound  dynamos  and  shunt  dynamos  can 
be  run  in  multiple  with  each  other  if  the  load  does 
not  vary,  or  if  the  latter's  field  is  regulated  by  hand; 
otherwise,  the  compound-wound  machine  will  take  more 
than  its  share  of  load  for  all  loads  above  that  at  which 
the  shunt  machine's  rheostat  has  been  adjusted.  This 
is  because  as  the  load  increases  the  shunt  machine's 
voltage  falls  off,  while  the  compound-wound  machine's 
-does  not.  Again,  if  the  shunt  machine  is  adjusted  to 
share  the  load  properly  at  full  load,  then  if  the  load 
decreases,  the  shunt  machine's  voltage  rises  while  that 
of  the  compound-wound  machine  falls,  if  the  latter  is  at 
all  overcompounded,  and  should  the  external  circuit  be 
opened,  leaving  the  two  machines  directly  connected  to 
each  other,  it  is  very  possible  that  the  shunt  machine  will 
reverse  the  compound-wound  machine,  and  run  it  as  a 
motor.  The  behavior  under  these  circumstances  is  very 
peculiar  if  the  two  machines  are  of  about  the  same 


THE    COMPOUND-WOUND    DYNAMO.  341 

capacity:  the  shunt  machine  starts  off  as  a  generator 
with  nothing  on  its  circuit  except  the  compound-wound 
machine's  field  and  armature.  The  current  in  the  com- 
pound-wound machine  has  been  reversed,  and  hence  the 
shunt  and  series  fields  oppose  and  neutralize  each  other, 
thus  destroying  all  C.  E.  M.  F.  that  the  machine  may  at 
first  produce.  The  shunt  machine,  now  acting  through 
a  short  circuit,  drops  its  field,  and  with  it  its  generating 
power.  The  compound-wound  machine  now  takes  a 
turn  as  generator,  and  is  in  turn  short  circuited  through 
the  shunt  machine's  armature.  By  virtue  of  its  series 
windings  the  compound-wound  machine  cannot  lose  its 
field,  and  unless  a  belt  or  circuit-breaker  gives  way,  will 
drive  the  shunt  machine  as  a  motor  and  at  a  high  rate 
of  speed.  Now  the  shunt  machine  runs  in  the  same 
direction,  with  given  connections,  whether  as  generator 
or  motor,  and  as  the  speed  rises,  so  does  the  C.  E.  M.  F., 
causing  the  armature  current  to  decrease,  and  with  it  the 
series  field  strength  on  the  compound-wound  machine 
till  the  latter  is  no  longer  able  to  maintain  its  voltage, 
and  is  again  reversed  and  run  as  a  motor.  The  same 
cycle  could  be  repeated,  if  circuit,  belts,  and  engines 
remain  intact. 

With  the  snunt  machine  separately  excited  it  cannot 
lose  its  field  at  any  stage,  and  the  behavior  is  somewhat 
modified;  at  the  point  where  the  compound-wound 
machine  is  run  as  a  motor,  the  current  in  its  series  field, 
being  reversed,  soon  overpowers  the  shunt  field  and 
reverses  its  polarity,  when  the  two  machines  now  run 
in  series  as  generators,  and  something  has  to  give  way 
or  burn  out.  Just  before  the  compound-wound  machine's 
polarity  is  reversed,  the  C.  E.  M.  F.  decreases  and 


342  TESTING    OF    DYNAMOS    AND    MOTORS. 

becomes  zero.  Until  this  point  is  reached  it  has  been 
running  as  a  shunt  motor,  and  hence  in  the  same  direction 
as  the  engine;  at  the  moment  of  the  reversal,  however, 
the  motor  action  of  the  armature  reverses  the  direction 
of  rotation,  and  at  this  point  a  belt  is  almost  sure  to 
break  or  fly  off.  Even  the  loss  of  a  belt  does  not 
permanently  relieve  matters  (except  in  special  cases 
where  the  belt  drives  a  loss  supplier),  for  the  two  arma- 
tures are  still  electrically  connected,  and  very  curious 
demonstrations  sometimes  take  place,  especially  with 
heavy  armatures  of  great  inertia.  Some  of  these  will 
be  dwelt  upon  in  treating  of  generator  tests. 

Compound-wound  machines  may  be  run  in  series  as 
well  as  multiple,  and  then  require  no  equalizer.  Like 
other  machines  they  are  placed  in  series  by  connecting 
any  two  terminals  that  will  enable  the  machines'  combined 
voltages  to  be  read  at  the  remaining  terminals.  When 
any  number  of  machines  are  in  series  the  same  current 
passes  through  all,  so  that  any  load  variations  must  be 
due  to  variations  in  voltage,  and  these  in  turn  are  due 
to  speed  variations  incidental  to  belt  slipping  and 
inequalities  in  driving  of  isolated  engines.  The  best 
regulation  is  secured  by  placing  the  shunt  fields  in  series,, 
and  exciting  them  with  the  entire  voltage  of  the  set;  in 
this  way  any  speed  variation  in  one  machine  affects  the 
fields  of  all,  so  that  the  effect  is  distributed. 

The  nicety  of  regulation  is  lost  when  compound-wound 
machine  and  shunt  or  separately  excited  machines  are 
run  in  series,  for  while  the  compound-wound  machine 
may  keep  the  voltage  constant  at  its  own  terminals,  it 
cannot  compensate  for  the  losses  in  the  machines  with 
which  it  is  in  series,  unless  especially  overcompounded 


THE    COMPOUND-WOUND    DYNAMO.  343 

to  do  so.  In  general,  therefore,  compound-wound 
machines  are  not  run  in  series  with  other  machines 
except  in  testing-room  practice,  and  here  it  is  either  for 
special  reasons,  or  through  force  of  circumstances.  It  is 
well,  however,  to  know  that  it  is  done,  for  though  one 
may  never  be  called  upon  to  run  such  a  test,  it  may  be 
convenient  to  adopt  some  feature  of  it. 

A  station  employing  only  compound-wound  machines 
may  be  regarded  as  a  single  compound-wound  machine 
of  variable  output,  and  may  be  put  to  service  on  the  same 
mains  with  another  similar  station,  provided  an  equalizer 
is  placed  between  the  stations  precisely  as  in  the  case  of 
two  machines,  for  otherwise  first  one  station  and  then 
the  other  will  take  the  greater  load.  There  is,  however, 
a  difference  in  degree,  though  not  in  kind,  between  the 
case  of  two  stations  and  that  of  two  dynamos.  Running 
two  stations  in  multiple  would  be  illustrated  by  dividing 
the  dynamos  of  one  large  station  into  two  sets,  all  those 
of  a  set  being  connected  by  an  equalizer,  but  each  set 
being  kept  distinct  from  the  other.  The  more  dynamos 
there  are  in  each  set,  the  smaller  will  be  the  percentage 
variation  of  load  per  machine  for  any  given  change  in  the 
station  output.  Since  it  is  the  per  cent,  change  which 
produces  a  change  in  the  voltage,  and  the  subsequent 
unbalancing  of  the  load,  it  follows  that  the  two  sets  could 
be  run  in  multiple  without  an  equalizer  more  satisfacto- 
rily than  two  single  dynamos  could  be.  Two  distant  sta- 
tions would  work  more  smoothly  together  in  proportion 
as  the  total  output  increased,  because  the  greater  the 
output  the  less  percentage  of  it  is  any  given  variation, 
and  hence  the  less  the  effect  upon  each  machine.  The 
fewer  the  dynamos  in  each  set,  the  more  marked  the 


344  TESTING    OF    DYNAMOS    AND    MOTORS. 

change,  and  a  limit  is  soon  reached  where  even  with  hand 
regulation  of  rheostats  it  is  impossible  to  keep  in  the 
individual  machine  circuit-breakers.  We  specify  individ- 
ual machine  circuit-breakers  because  the  latter  can  be 
disposed  in  either  of  two  ways:  each  machine  may  have 
a  circuit-breaker,  or  there  may  be  a  circuit-breaker  set  at 
their  combined  output,  and  situated  in  the  bus-bar  com- 
mon to  all  the  machines,  in  the  first  case,  as  soon  as  a 
machine  for  any  reason  takes  undue  load,  its  own  breaker 
responds;  in  the  second  case,  even  if  the  equalizer  is  in 
use,  the  machines  are  not  protected,  and  are  free  to 
reverse  each  other  should  there  be  any  great  discrepancy 
in  their  voltages,  and  the  external  load  becomes  greatly 
reduced.  This  is  because  where  the  external  load  is 
great  the  external  resistance  is  comparative!^  low  and 
offers  an  easy  path  to  the  output  of  the  machines  in 
common;  but  if  the  load  gets  small,  or,  to  take  an  ex- 
treme but  common  case,  if  the  load  becomes  zero,  as  it 
does  when  either  the  consumers  cease  to  use  power, 
or  the  station  circuit-breaker  goes  out,  there  is  no 
common  outside  circuit  for  the  machine's  currents  to 
concur  in,  and  they  are  left  to  oppose  each  other  in  the 
local  circuits  of  the  machines  themselves.  This  point 
and  its  bearing  on  station  practice  is  considered  else- 
where. The  more  highly  compounded  the  dynamos 
are,  the  more  violent  the  demonstration  upon  trying  to 
run  them  without  an  equalizer,  because  so  much  the  more 
rapidly  does  a  machine's  voltage  run  up  when  the  current, 
and  hence  the  series  field  strength,  increases.  However, 
instances  could  be  cited  where  two  stations  have  been 
successfully  run  in  multiple  without  an  equalizer,  but  in 
these  cases  the  stations  were  several  miles  apart,  and  the 


THE    COMPOUND-WOUND    DYNAMO.  345 

comparatively  high  resistance  of  the  overhead  work  and 
ground  return  made  the  feat  practicable  by  narrowing 
the  limits  between  which  fluctuations  could  take  place. 
To  see  this  more  clearly,  suppose  the  two  stations  to  be 
10  miles  apart,  and  that  the  load,  contributed  to  by  both 
in  common,  is  midway  between  them;  let  the  line  resist- 
ance between  the  two  stations  be,  say,  2  ohms.  Now 
since  any  difference  in  load  taken  by  the  two  similarly 
constructed  stations  is  due  to  a  difference  in  their 
E.  M.  Fs.,  any  one  station  will  take  its  maximum  load 
when  this  difference  in  E.  M.  F.  is  greatest,  and  this 
will  be  the  case  when  the  E.  M.  F.  of  the  opposing 
station  is  zero.  Call  one  station  A  and  the  other  />',  and 
suppose  y/s  effective  voltage  upon  the  line  to  become 
zero.  This  condition  can  be  obtained  in  either  of  two 
ways:  i.  By  opening  ^'s  line  switch  and  cutting  B  out 
of  circuit;  2.  In  event  of  the  driving  power  of  B  be- 
coming suddenly  disabled,  as  by  bursting  a  steam  pipe, 
throwing  a  belt,  or  breaking  a  shaft  or  turbine,  thereby 
leaving  /?'s  dynamos  inactive  electrically,  but  directly 
across  the  line  as  a  short  circuit.  In  the  first  case  the 
maximum  current  which  A  can  send  depends  upon  R, 
the  resistance  in  the  path  of  the  natural  load  plus  the 
line  resistance  from  A  to  R  (in  this  case  i  ohm),  and 
even  if  R  becomes  short  circuited  the  current  cannot 
exceed 

600 

-  =  600  amperes, 


where  600  is  A's  E.  M.  F.  and  i  ohm  =  R.  In  the 
second  case,  where  we  suppose  an  accident  to  leave  B  a 
short  circuit  across  the  line,  the  maximum  current  which 


346  TESTING    OF    DYNAMOS    AND    MOTORS. 

A  can  be  called  upon  to  generate  is,  if  we  neglect  the 
natural  load  taken  by  the  devices  midway  between  the 
two  stations, 

E,       600 

—  =  -  -  =  300  amperes. 


We  see,  then,  the  limiting  effect  which  the  line  resistance 
has  upon  the  current  under  these  extreme  conditions.  It 
is  this  effect  which,  as  it  were,  cushions  the  variations 
due  to  working  inequalities,  and  makes  parallel  working 
of  distant  stations  practicable  without  the  equalizer. 

All  direct  current  generators  of  whatever  kind  may  be 
run  in  series  with  each  other,  any  difference  in  voltage 
making  no  difference  in  this  regard.  A  500  volt  street 
railway  generator  could  be  connected  in  series  with 
a  Daniell  cell,  to  give  501  volts  without  injury  to 
either.  The  question  of  the  possibility  of  series  running 
is  a  question  merely  of  direction  of  voltage,  so  that 
running  two  500  volt  machines  in  series  is,  in  principle, 
the  same  if  one  of  them  be  replaced  by  a  Daniell 
cell.  The  practical  drawback  to  utilizing  such  a  com- 
bination as  a  source  of  current  is  the  discrepancy  in 
current  capacity  between  the  two  devices;  it  is  even 
unwise  to  run  in  series  machines  differing  greatly  in 
current  capacity,  lest  the  one  of  lower  capacity  be 
dangerously  overloaded.  In  the  effort  to  gain  other 
ends,  this  point  is  apt  to  be  overlooked,  especially  in 
testing  rooms  where  so  many  armatures  of  the  same  out- 
put but  of  different  E.  M.  Fs.  are  run  in  the  same 
separately  excited  frames.  This  error  is  well  illustrated 
in  outside  practice  by  the  superintendent  operating  a 
three-wire  lighting  plant  who  returned  to  the  factory 


THE    COMPOUND-WOUND    DYNAMO.  347 

a  125  volt  ioo  KW  armature  to  be  rewound;  the  one 
returned  to  him  was  wound  for  250  volts,  but  in 
general  appearance  was  the  same  as  the  old  one.  The 
state  of  affairs  existing  on  the  line  as  soon  as  the  250 
volt  armature  running  in  125  volt  fields  joined  forces 
with  its  125  volt  mate,  would  have  been  deplorable  had 
the  line  switch  been  thrown  in,  but  a  smoking  rheostat 
attracted  attention,  and  excited  suspicion,  and  upon 
shutting  down  some  quick  eye  noticed  that  the  new  com- 
mutator had  more  bars  than  the  old  one. 

The  practice  of  running  storage  batteries  and  dyna- 
mos together  is  not  without  illustration,  especially  in 
European  power  and  lighting  station  practice,  where 
batteries  carry  much  of  the  day  load,  and  also  the  peak 
of  the  load  when  running  at  full  capacity;  in  this  way  the 
machine  capacity  required  in  the  station  is  reduced,  and 
a  higher  average  load  secured,  so  that  the  station  runs 
at  greater  efficiency.  The  factors  modifying  efficiency 
are  considered  elsewhere  in  this  book,  but  as  a  natural 
sequence  to  the  above  assertion  that  "the  station  runs 
at  greater  efficiency,"  we  will  say  it  is  because,  in  all 
devices  for  converting  or  transmitting  energy,  there  are 
certain  losses  due  to  radiation  of  heat,  condensation  of 
steam,  friction  of  moving  parts,  resistance  of  electrical 
conductors,  and  what  not.  If  a  station  runs  without 
delivering  any  work  to  consumers,  the  internal  losses 
constitute  the  entire  station  load,  there  is  no  output  to 
make  returns,  and  the  station  efficiency  is  zero.  If  on 
the  other  hand  the  station  runs  at  full  load,  the  losses 
increase,  it  is  true,  but  not  nearly  in  the  proportion  that 
the  useful  output  which  is  being  paid  for  does.  Again, 
if  a  station  is  equipped  to  deliver  1,000  horse  power  to 


34^  TESTING    OF    DYNAMOS    AND    MOTORS. 

consumers,  and  it  is  called  upon  to  deliver  only  500  horse 
power,  it  means  that  the  station  is  twice  too  large,  and 
that  half  the  money  invested  might  better  be  elsewhere 
drawing  interest. 

The  object  sought  in  series  running  is  generally  a 
higher  voltage  than  is  obtainable  from  a  single  machine, 
and  cases  are  on  record  where  the  voltage  on  arc  cir- 
cuits has  been  run  up  to  6,000  or  7,000  volts.  The  rea- 
son such  voltages  are  not  attainable  on  single  machines 
is  that  the  insulation  will  not  stand  it,  especially  that  of 
the  commutator.  By  connecting  several  machines  in 
series,  each  machine's  insulation  is  subjected  only  to  the 
potential  difference  Between  that  machine's  terminals, 
unless  one  side  of  the  line  becomes  grounded,  then  the 
machine  nearest  the  other  side  the  line  is  subjected  to 
the  full  voltage  of  the  system,  if  the  frames  of  all  the 
machines  are  metallically  connected,  as  they  would  be  on 
an  iron  floor.  But  this  is  seldom  the  case,  and  the  result 
of  a  ground  on  any  machine  is  simply  to  weaken  the 
breaking  down  insulation  of  that  machine.  If  with  any 
number  of  machines  in  series  it  is  desired  to  cut  one  or 
more  out  of  service,  it  is  easily  done  by  reducing  the 
machine's  field  to  zero.  On  shunt  machines  this  is  done 
by  opening  the  shunt  field  circuit  by  means  of  the 
rheostat  and  a  switch;  on  series  machines  by  short 
circuiting  the  series  field.  In  either  case  there  is  no 
danger  of  the  machine's  running  as  a  motor  or  giving  any 
trouble.  Where  a  shunt-  and  a  compound-wound  dynamo 
run  in  series,  and  it  is  desired  to  remove  one  from  ser- 
vice, it  is  best  to  remove  the  shunt  machine,  for  although 
the  compound-wound  machine's  shunt  field  may  be  open, 
the  series  field  still  remains,  and  the  dynamo  generates 


THE    COMPOUND-WOUND    DYNAMO.  349 

until  the  series  coils  are  short  circuited.  In  cutting  out 
a  machine  gradually  weaken  its  field  before  breaking  it, 
because  aside  from  injury  liable  to  result  from  the 
inductive  discharge  upon  suddenly  breaking  a  field,  very 
curious  and  inconvenient  phenomena  arise  if  the  circuit 
contains  a  shunt-  or  compound-wound  motor  of  great 
inertia.  These  phenomena  are  detailed  in  Chapter  IX. 
The  reason  that  weakening  the  field  of  a  machine  in  series 
with  others  does  not  make  it  liable  to  be  run  as  a  motor, 
is  as  follows:  so  long  as  there  is  any  field  of  the  same 
polarity  as  at  full  load,  the  direction  of  the  E.  M.  F. 
generated  by  the  armature  remains  unchanged,  hence 
concurs  with  and  contributes  to  the  line  E.  M.  F.  As 
soon  as  the  field  is  broken  the  armature  may  generate  a 
slight  E.  M.  F.  in  virtue  of  the  residual  field,  or  in  virtue 
of  the  slight  influence  which  the  armature  current  may 
have  upon  the  pole  pieces.  These  two  effects  are 
opposed  and  tend  to  neutralize  each  other,  so  neglecting 
their  resultant,  we  must  consider  the  moving  armature 
to  be  electrically  inert,  and  to  act  purely  as  any  other 
ohmic  resistance  in  circuit,  and  to  consume  energy  in 
the  form  of  heat.  Since  this  energy  is  consumed  as 
heat,  it  is  unavailable  for  turning  the  armature  as  a 
motor,  and  is  so  small  an  amount  that  it  could  not  do  so 
were  it  available.  To  run  the  machine  as  a  motor,  a  cer- 
tain amount  of  electrical  energy  must,  within  the  machine 
itself,  be  transformed  into  the  mechanical  energy  of  rota- 
tion, and  to  secure  this  transformation  there  must  be  a 
greater  potential  difference  at  the  terminals  than  that 
due  solely  to  ohmic  resistance.  The  energy  expended 
in  any  part  of  a  circuit  is  equal  to  the  product  of  the  cur- 
rent flowing,  by  the  potential  difference  at  the  terminals; 


350  TESTING    OF    DYNAMOS    AND    MOTORS. 

if  this  potential  difference  is  entirely  due  to  ohmic  resist- 
ance, the  energy  is  expended  entirely  as  heat:  if,  how- 
ever, part  of  it  be  due  to  the  presence  of  a  C.  E.  M.  F., 
a  portion  (equal  to  /  x  C.  E.  M.  F. )  will,  under  proper 
conditions,  be  transformed  into  mechanical  energy.  The 
conditions  are  that  the  armature  be  free  to  mo.ve  in  a 
magnetic  field.  But,  the  reader  will  say,  the  last  condi- 
tions are  fulfilled  when  the  armature  runs  as  a  generator; 
true,  it  is  free  to  turn  and  has  a  field  to  turn  in,  but  the 
.limiting  condition  is  this:  the  machine's  polarity  must 
oppose  that  of  the  machine  or  line  with  which  it  is  run- 
ning; its  E.  M.  F.  then  becomes  a  C.  E.  M.  F.,  and  only 
then  can  the  machine  act  as  a  motor,  because,  to  retro- 
.spect  a  little,  we  can  state  that  according  as  a  machine 
runs  as  a  dynamo  or  a  motor,  so  will  the  relation  exist- 
ing between  motion  and  repulsion  vary  also.  When  an 
engine-driven  armature  turns  in  a  magnetic  field  and  is 
free  to  generate  current,  there  acts  between  this  current 
and  the  magnetic  field  an  attractive  or  repulsive  force,  as 
we  may  choose  to  call  it,  which  resists  the  effort  to  turn 
the  armature,  and  it  is  turning  the  armature  around 
against  this  force  that  causes  the  engine  to  do  work;  in 
this  case  the  repulsion  is  due  to  the  motion.  When 
a  current  is  sent  through  an  armature  standing  in  a  mag- 
netic field,  the  force  acting  between  the  current's  lines 
of  force  and  those  of  the  field  causes  the  armature  to 
turn;  in  this  case  the  motion  is  due  to  the  repulsion,  and 
we  have  the  motor.  As  soon  as  the  motor  armature 
begins  to  turn,  its  conductors  cut  the  field's  lines  of  force 
and  generate  an  E.  M.  F.  opposed  to  that  which  causes 
the  armature  to  turn,  and  is  hence  called  a  C.  E.  M.  F. 
(That  this  E.  M.  F.  is  opposed,  why  it  is  opposed,  and 


THE    COMPOUND-WOUND    DYNAMO.  351 

that  an  armature  constructed  to  have  no  C.  E.  M.  F. 
would  be  incapable  of  motion  is  taken  up  in  another 
chapter.)  Therefore,  if  by  means  of  a  steam  engine  we 
cause  a  machine  to  generate  an  E.  M.  F.  which  it  is 
desired  to  utilize  as  a  C.  E.  M.  F.,  the  polarities  of 
the  machine  in  question  and  that  of  the  line  or  other 
machine  must  be  opposed;  otherwise  the  voltages  con- 
cur in  direction,  and  the  machine  will  take  up  its  work  as 
a  generator. 

In  running  dynamos  in  series  it  often  happens  that 
they  are  run  from  different  engines,  or  that  one  has  a 
smaller  belt  than  is  intended  for  it;  under  these  circum- 
stances it  is  possible  for  one  engine  to  become  over- 
loaded, or  for  undue  tension  on  the  smaller  belt  to  cause 
it  to  slip.  To  afford  relief,  it  is  only  necessary  to 
weaken  the  field  on  the  machine  in  question;  since  the 
current  is  the  same  in  all  the  machines,  the  load  on  each 
is  proportional  to  its  E.  M.  F. ,  and  hence  decreasing  this 
will  decrease  the  load.  The  ease  with  which  machines 
of  widely  varying  voltage  can  be  run  in  series  facilitates 
the  testing  of  dynamos  of  low  voltage  but  large  current 
capacity.  In  this  case,  the  two  machines  should  be  of 
about  the  same  current  capacity,  and  the  low  voltage 
machine  may  be  run  as  a  motor  to  share  the  load  with 
the  lamp  bank  or  water  box,  or  it  may,  as  a  dynamo  in 
series  with  the  larger  machine,  furnish  a  small  voltage 
and  help  urge  the  required  current  through  the  lamp 
bank,  water  box,  or  motor,  as  the  case  may  be. 

TEST  I. — Eight-volt,  five-ampere  Shunt  Machine. — Low 
voltage  generators  for  electroplating  purposes  give  from 
5  to  25  volts,  and  from  10  amperes  upward;  machines 
which  are  to  be  used  as  "  boosters  "  are  also  of  low  voltage 


352  TESTING    OF    DYNAMOS    AND    MOTORS. 

and  high  amperage.  In  any  case  it  is  difficult  to  get  a 
resistance  low  enough  to  admit  of  the  low  voltage  send- 
ing the  required  current  through  it,  and  at  the  same  time 
of  sufficient  capacity  to  carry  the  current.  In  the  case 
of  very  large  current  capacities  the  voltage  is  even  lower, 
and  the  test  is  conducted  by  stringing  copper  cables 
from  brush  to  brush.  When  neither  lamp  bank  nor 
rheostat  is  available,  it  is  customary  to  utilize  a  third 
machine  as  motor,  and  to  have  it  return  its  energy  of 
rotation  to  the  original  motive  power. 
(This  method  of  testing,  known  as 
"pumping  back,"  is  of  wide  applica- 
tion, is  an  important  factor  in  testing 
room  economy,  and  is  considered  by 
itself  elsewhere.)  In  the  above  test 
of  a  low  voltage  machine  we  will 
FIG  Lii3  suppose  that  the  machine  is  to  be 

run  as  a  generator  in  series  with 
an  auxiliary  dynamo  of  equal  current  capacity  but 
of  much  higher  E.  M.  F.,  so  that  their  combined 
E.  M.  Fs.  will  be  sufficient  to  urge  the  required  current 
through  the  resistance  of  a  lamp  bank.  Low  voltage 
machines  are  generally  separately  excited,  so  they  can 
readily  be  rendered  active  or  idle  by  making  or  breaking 
their  field.  Let  us  suppose  that  we  have  an  8  volt  25 
ampere  machine  to  run  for  2  hours,  and  that  the  machine 
to  be  used  as  an  auxiliary  is  a  125  volt  48  ampere 
machine.  As  a  preliminary,  the  two  machines  are  run 
with  a  light  field  and  gotten  in  series  without  closing 
switch  K,  of  Fig.  113;  where  A  is  the  auxiliary  machine, 
B  the  one  under  test,  C  and  D  machines  or  lines  to  be 
used  as  exciters,  and  L  the  lamp  bank  filled  with  85  volt 


THE    COMPOUND-WOUND    DYNAMO.  353 

lamps.  It  is  not  absolutely  essential  that  cither  A  or  B 
be  separately  excited,  for  the  test  has  been  frequently 
run  with  both  machines  self-excited,  when  exciters  were 
not  available,  but  all  testers  have  a  preference  for  sepa- 
rate excitation,  because  it  admits  of  such  easy  control, 
and  there  is  no  danger  of  a  machine  losing  its  field  by 
increasing  its  field  circuit  resistance.  In  the  diagram 
F  is  A's  field  excited  from  C,  and  F\  J3's  field  excited 
from  D,  and  containing  a  switch  at  K ' .  The  first  step  is 
to  get  the  two  machines  in  series;  putting  a  light  field 
on  both  machines,  read,  by  means  of  a  \foilmeter,  the 
voltage  from  i  to  2,  and  that  from  3  to  4;  the  voltage 
from  i  to  4  should  be  their  sum;  if  it  proves  to  be  their 
difference  it  shows  the  machines  to  be  in  opposition,  and 
to  place  them  in  series  the  polarity  of  one  of  them  must 
be  reversed.  In  the  above  case  this  is  readily  done  by 
reversing  the  field  of  one  machine,  but  if  both  A  and  B 
are  self-exciting,  it  will  be  necessary  to  reverse  their 
line  terminals;  because  if  a  self-exciting  dynamo  have 
either  its  field  or  armature  reversed,  it  will,  for  reasons 
to  be  seen  later,  refuse  to  generate.  Reversing  the  line 
terminals  reverses  both  field  and  armature,  and  preserves 
their  relation  to  each  other. 
This  can  be  seen  from  Fig.  114, 
where  if  i  is  put  where  2  is,  and 
2  where  i  is,  both  A  and  F  are 
reversed.  The  next  step  is  to 
break  .Z?'s  field,  close  A",  and 

by  means  of  A  alone  get  the  required  current  (25 
amperes)  on  the  lamp  bank,  so  as  to  get  an  approxi- 
mate idea  of  where  A's  rheostat  is  to  rest,  and  of 
the  condition  of  the  lamp  bank,  and  how  it  must  be 


354  TESTING    OF    DYNAMOS    AND    MOTORS. 

plugged;  next,  without  disturbing  the  position  of  A's 
rheostat,  throw  off  the  load  by  opening  A",  and  ob- 
serve the  voltage  on  open  circuit;  say  it  goes  up  to  90 
volts;  we  know  then  that  in  order  to  get  25  amperes 
through  Z,  plugged  as  it  is,  the  combined  open  circuit 
voltage  of  the  two  machines  must  be  90  volts.  This  data 
once  secured,  the  field  on  A  can  be  weakened  a  little,  K' 
closed,  the  voltmeter  placed  across  B,  K  closed,  and  A's 
and  £'s  field  rheostats  varied  simultaneously  until  £'s 
voltage  is  8  and  the  current  is  25  amperes,  the  conditions 
sought. 

In  case  the  low  volt  machine  is  series-wound  and  self- 
exciting  no  adjustment  of  its  terminal  voltage  can  be 
made  unless  it  is  too  high,  when  an  improvised  shunt 
can  be  used  to  reduce  it;  the  current  is  then  the  only 
quantity  to  be  watched.  In  any  case,  as  a  last  resort, 
the  size  of  the  pulley  can  be  changed  to  give  a  speed 
that  will  meet  the  requirements  of  the  voltage.  Varia- 
tions can  be  gotten  within  certain  limits,  by  rocking  the 
brushes,  forward  to  raise  and  backward  to  lower  the 
voltage;  such  a  variation  being  due  to  alteration-  in 
armature  reaction. 

In  using  a  high  voltage  auxiliary  machine  in  connec- 
tion with  a  lamp  bank,  or  several  banks  in  series,  care 
must  be  exercised  in  plugging  the  banks,  or  they  may 
suffer  injury.  Lamp  banks  are  generally  arranged  so 
that  they  can  either  be  connected  in  series,  and  the 
lamps  plugged  in  multiple,  or  connected  in  multiple  to 
plug  the  lamps  in  series.  In  plugging  or  unplugging 
banks  connected  in  series,  keep  as  far  as  possible  the 
same  number  of  burning  lamps  in  each  bank,  for  if  lamps 
are  cut  out  of  one  bank  faster  than  out  of  another  in 


THE    COMPOUND-WOUND    DYNAMO.  355 

series  with  it,  the  resistance  of  the  bank  being  increased 
while  the  current  is  not  proportionately  decreased,  the 
fall  of  potential  through  the  remaining  lamps  (equal  to 
/  R)  is  increased  above  normal  value,  and  may  burn 
them  out.  In  other  words,  by  removing  lamps  from  one 
of  a  series  of  banks  the  current  carrying  capacity  of  that 
part  of  the  circuit  becomes  too  small  to  carry  the  cur- 
rent, and  is  burned  out.  As  an  extreme  case,  suppose 
that  in  one  of  three  banks  of  TOO  lamps  each,  50  have 
been  cut  out;  if  we  call  the  resistance  of  each  full  bank 
r,  and  the  total  E.  M.  F.  applied  £,  we  have  when  all 
the  banks  are  full, 


After  cutting  out  half  the  lamps  in  one  bank  its  resist- 
ance becomes  2  r,  all  the  rest  remaining  the  same  as 
before:  then, 


2  r  +  r  +  r         4  r 
In  the  first  case  each  bank  gets  a  potential  difference  of 

A, 
3 

because  each  one's  resistance  is  one-third  of  the  total. 
In  the  second  case  the  total  is  4  r,  and  that  of  the  50- 
lamp  bank  2  r,  or  one-half  the  total  resistance.  This. 
bank  therefore  is  subjected  to  one-half  E,  which  is 
too  much  for  the  lamps.  For  the  same  reason,  all  the 
rows  of  a  bank  should  be  full  of  lamps.  The  first  indi- 
cation of  missing  lamps  is  when  those  remaining  in  the 


356  TESTING    OF    DYNAMOS    AND    MOTORS. 

row  shine  out  more  brightly  than  the  rest.  There  are 
many  types  of  lamp  bank;  one  of  those  most  used  is 
known  as  the  three-wire  lamp  bank.  It  can  be  plugged 
in  any  series  multiple  combination  within  its  capacity. 

TEST  II. — Motor  Generator  Test,  Engine  as  Loss  Supplier. 
— In  the  whole  range  of  testing  room  expenence  there  is 
no  test  so  instructive  as  the  ''motor-generator"  test 
alluded  to  above.  From  a  scientific  standpoint  it  is  one 
of  the  neatest  applications  of  theory  to  practice  that 
can  be  found,  and  admirably  illustrates  the  flexibility 
of  the  electrical  system  of  energy  transmission.  From 
an  economic  standpoint,  the  coal  dealer  frowns  in  evi- 
dence of  its  effectiveness.  The  method  is  an  elabora- 
tion of  Drs.  J.  and  E.  Hopkison's  "  Efficiency  Test," 
and  though  carried  out  in  several  ways  differing  in 
detail,  is  in  principle  as  follows:  A  dynamo  belted  to  an 
engine  is  electrically  connected  to  a  motor  also  belted 
to  the  engine;  the  electrical  energy  of  the  dynamo  goes 
into  the  motor,  there  to  be  converted  into  the  mechani- 
cal energy  of  rotation,  which  in  turn  helps  the  engine  to 
turn  the  dynamo.  If  there  were  no  losses  due  to  heat, 
friction,  belt  tension,  etc.,  the  motor  and  dynamo,  once 
started,  would  continue  to  run  each  other  without  ?.ny 
help  from  the  engine;  but  we  know  that  the  dynamo  can 
deliver  only  a  part  of  its  own  energy  to  the  motor,  and 
that  the  motor  can  return  only  a  part  of  what  it  receives 
to  the  dynamo;  in  both  machines,  in  the  engine  itself, 
and  in  the  shafting  there  is  energy  lost,  and  it  is  this 
lost  energy  which  the  engine  or  auxiliary  motor  or 
dynamo,  as  the  case  may  be,  must  supply  in  order  to 
keep  the  system  turning. 

In  large  factories  where  many  machines  of  large  out- 


THE    COMPOUND-WOUND    DYNAMO. 


357 


put  are  tested  daily,  this  most  economical  method  is 
adopted,  and  in  the  following  pages  we  give  the  test 
with  the  various  modifications  dictated  by  circumstances. 
The  test  in  its  simplest  form  is  conducted  on  two  elec- 
trically connected  machines  belted  to  an  engine,  which 
is  to  supply  the  loss.  In  this  case,  the  same  amount  of 
current  flows  through  both  machines,  and  they  must  be 


FIG.  115. 

of  very  nearly  the  same  output,  or  the  full  current  load 
of  one  will  overload  the  other.  Let  the  test  be  of  two 
200  KW  500  volt  machines,  shunt  wound,  and  belted  to 
a  75  KW  engine  to  supply  the  loss!  In  Fig.  115,  Pis 
the  engine  pulley,  P\  P\  are  the  countershaft  pulleys, 
to  which  are  belted  armatures  A  and  B,  by  means  of  their 
respective  pulleys,  6"  and  S" .  F  and  R  are  A's  field 
and  rheostat,  F'  and  R'  those  of  B.  G  is  an  ammeter, 
and  K  a  switch  between  the  two  machines.  It  can  be 
seen  that  one  of  A's  brushes  connects  directly  to  one  of 
It's  through  cable  i,  2,  3,  and  that  the  two  remaining 


358  TESTING    OF    DYNAMOS    AND    MOTORS. 

brushes  connect  through  cable  4,  5,  6,  including  meter 
G  and  switch  K,  so  that  the  circuit  through  the  two 
machines  is  i,  2,  3,  4,  5,  6.  The  field  of  each  machine 
is  connected  to  its  brushes  or  their  equivalent,  and  each 
can  generate  its  own  field,  even  though  K  be  open. 
A's  field  circuit  is  i,  ^?,  F,  6;  and  B's,  3,  R'<  F\  4. 
After  completing  connections,  everything  is  inspected  to 
see  that  pulleys  are  tight,  brushes  are  set,  bearings  are 
oiled,  and  that  connections  are  proper  for  the  given 
direction  of  rotation.  The  engine  is  then  turned  over 
slowly  to  facilitate  lining  up  the  pulleys,  and  getting  the 
right  belt  tension.  It  is  then  brought  up  to  speed,  and 
if  the  armature  pulleys  have  been  properly  selected  they 
too  will  run  at  their  rated  speed.  The  next  step  is  to 
let  down  one  machine's  brushes,  close  its  field  circuit, 
get  a  field,  and  by  means  of  a  voltmeter  and  rheostat,  R, 
adjust  the  voltage  to  500.  We  then  hold  the  volt  lines 
across  the  rheostat  terminals,  and  get  what  is  called 
the  drop  on  the  box.  This  data  tells  the  engineer  if  the 
quality  of  the  iron  is  what  it  should  be,  because  if  the 
iron  is  poor  and  everything  else  is  all  right,  it  will  take 
more  current  in  the  field  circuit  to  give  the  required 
voltage;  to  get  more  field  current  the  box  resistance 
must  be  lessened,  and  this  lessens  the  drop  on  the  box. 
We  now  raise  .Z?'s  brushes  and  close  K\  this  allows  A  to 
charge  £'s  field,  so  that  when  B  generates,  its  voltage 
will  oppose  that  of  A.  K\§  now  opened,  and  It's  brushes 
lowered,  when,  if  everything  is  all  right,  B  will  pick  up 
and  support  its  own  field;  its  voltage  also  can  now  be 
adjusted  to  500,  when  the  following  state  of  affairs  will 
exist :  A  and  B  are  generating  equal  but  opposite  voltages, 
so  that  a  voltmeter  across  K  will  not  be  deflected;  when 


THE    COMPOUND-WOUND    DYNAMO.  359 

this  is  the  case,  K  may  be  closed,  and  no  current  will 
flow  between  A  and  B,  because  they  have  equal  but  oppo- 
site tendencies  to  send  current  into  each  other.  A'closed, 
the  system  is  ready  for  load.  One  machine,  say  A  (it  is 
immaterial  which  when  testing  shunt  machines),  is  selected 
to  be  the  motor;  B,  to  be  the  generator.  Placing  a  man 
at  ^Ts  brushes,  one  at  B's  brushes,  and  another  to  tighten 
the  belt,  if  necessary,  as  the  load  goes  on,  As  field  is 
gradually  weakened.  B,  against  A's  diminished  E.  M.  F., 
now  sends  current  through  A  and  runs  it  as  a  motor;  its 
former  E.  M.  F.  becoming  its  C.  E.  M.  F.  As  load  gees 
on,  A's  brushes  must  be  brought  backward  to  stop  their 
sparking,  and  ^?'s  forward.  (The  effect  of  careless  hand- 
ling of  brushes  in  a  motor-generator  test  is  detailed  else- 
where.) With  the  voltmeter  across  />"s  terminals,  .Z>'s 
voltage  is  adjusted,  by  means  of  ./?',  to  500  volts.  The 
speed  is  checked  up,  and  the  full  load  drop  on  the  box 
taken.  The  machines  are  then  run  half  the  stipulated  time 
of  the  test,  when  they  are  changed  over.  A  becoming 
the  generator  and  B  the  motor.  To  do  this,  A*  field  is 
very  gradually  strengthened,  .Z?'s  weakened,  A's  brushes 
rocked  forward  and  j£'s  backward,  till  the  ammeter,  Gt 
indicates  zero;  continuing  the  operations  carefully  the 
needle  will  rise  to  full  load  again. 

The  first  few  tests  are  free  from  details  concerning 
data,  precautions,  troubles,  symptoms,  and  remedies, 
because  it  is  thought  best  not  to  burden  or  side  track  the 
reader's  attention,  till  his  conception  is  clear  as  to  their 
first  principles.  These  matters  will  be  entered  into  more 
fully  later. 

TEST  III.—  Motor  Generator  Test  with  Lamp  Bank.—^t 
test  just  considered  comprised  two  machines  of  equal  volt- 


3<5° 


TESTING    OF    DYNAMOS    AND    MOTORS. 


250V 


age  and  equal  current  capacity.  We  now  take  up  a  test  in- 
volving  two  machines  of  approximately  the  same  current 
capacity  but  of  very  different  E.  M.  Fs. ,  the  ' '  loss  "  to  be 
supplied  by  an  engine  to  which  both  machines  are  belted. 
The  two  machines  can  of  course  be  run  on  a  lamp  bank  or 
water  box,  but  aside  from  the  economic  advantage  of  hav- 
ing one  machine,  as  a  motor,  return  its  energy  to  the  sys- 
tem, we  shall  suppose,  as  is  often  the  case,  that  the  engine 

available  is  already  nearly 
loaded,  or  is  too  small  to 
support  either  machine 
run  as  a  dead  load  on  a 
lamp  bank.  In  this  case 
we  will  use  a  lamp  bank, 
in  series  with  the  smaller 
machine,  to  absorb  the 
excess  of  voltage  of  the 
larger  one.  Fig.  116  gives 

the  connections.  A  is  the,  say,  125  volt  machine  in 
series  with  lamp  bank  Z,  filled  with  no  or  50  volt 
lamps.  B  is  a  250  volt  machine,  which  must  be  the 
generator  in  this  case,  for  we  could  never,  without 
separately  exciting  it,  get  its  voltage  enough  below 
that  of  A  to  use  it  as  a  motor.  From  the  diagram 
it  is  seen  that,  as  far  as  connections  go,  the  two 
machines  and  the  lamp  bank  are  in  series;  but  inas- 
much as  the  E.  M.  Fs.  of  A  and  B  are  opposed  in  the 
test,  the  question  arises:  Are  A  and  B  in  series?  Good 
authorities  differ  on  this  point.  Kapp  in  his  "  Electrical 
Transmission  of  Energy  "  refers  to  a  motor  and  dynamo 
being  in  series,  and  using  this  as  a  basis,  the  matter  may 
as  be  reasoned  out  thus:  For  two  dynamos  to  be  in  series 


THE    COMPOUND-WOUND    DYNAMO.  361 

their  E.  M.  Fs.  must  concur,  and  for  two  motors  their 
C.  E.  M.  Fs.  must  concur.  In  the  test  under  considera- 
tion, both  machines  are  brought  up  to  full  voltage  before 
the  circuit  between  them  is  closed.  They  are  therefore 
primarily  dynamos,  although  they  generate  only  enough 
current  to  excite  their  own  fields.  If,  now,  we  suppose 
their  E.  M.  Fs.  to  be  equal,  so  that  the  voltmeter  across 
K  reads  zero,  when  the  switch  is  closed  the  machines  are 
still  dynamos,  and  are  in  multiple;  if  the  field  of  one  be 
weakened  and  it  becomes  a  motor,  its  nature  is  changed 
and  the  two  machines,  as  in  the  above  case,  are  in  series. 
To  start  the  test,  the  engine  is  brought  up  to  speed 
and  the  speed  of  the  two  machines  taken  to  insure  that 
the  right  pulleys  have  been  selected.  The  field  is  now 
put  on  B,  and  its  voltage  reduced  so  as  not  to  injure  A's 
fields  when  charging  them.  To  charge  ^'s  field,  first 
raise  its  brushes  and  close  its  headboard  switch  if  it 
has  one.  Machines  do  not  always  have  their  switches 
at  the  time  of  testing,  but  if  in  this  case  both  of  them 
have,  one  of  the  switches  can  take  the  place  of  A",  the 
other  remaining  permanently  closed.  A's  brushes  raised, 
K  is  closed  and  L  short  circuited  by  means  of  plug  K', 
plug  S,  of  course,  being  in  to  close  the  circuit.  Particu- 
lar care  must  be  exercised  to  get  the  two  machines  in 
opposition,  and  not  in  series  as  generators,  for  in  the 
latter  case,  should  K  and  6"  be  closed  and  K'  open,  the 
lamps  would  be  destroyed,  as  many  a  novice  can  testify. 
With  K,  K',  and  S  closed,  and  A  and  B  in  series,  a  short 
circuit  would  follow  and  the  belts  would  fly  off.  Having 
charged  A's  field,  K  is  opened,  K  is  opened  also,  and 
A's  brushes  lowered.  A  should  now  generate  its  own 
field.  Next  draw  out  S,  and  close  K',  S  then  becomes 


362  TESTING    OF    DYNAMOS    AND    MOTORS. 

the  only  point  at  which  the  circuit  between  A  and  B  is 
broken,  and  the  E.  M.  F.  across  it  should  be  the  differ- 
ence between  the  E.  M.  Fs.  of  A  and  B.  Should  it  be 
their  sum,  it  shows  the  machines  to  be  in  series,  and  A's 
field  must  be  recharged.  The  writers  do  not  recall  an 
instance  of  a  machine's  reversing  its  polarity  immediately 
after  charging,  but  as  a  tester  sometimes  forgets  to 
charge,  leaving  the  machine  to  pick  up  as  it  may,  the 
test  across  K  is  a  necessary  one.  Always  be  careful  to 
open  K  or  S  before  lowering  the  brushes,  and  to  remove 
K  before  completing  the  circuit  again,  for  either  over- 
sight will  result  in  a  short  circuit. 

With  the  machines  of  opposing  voltage,  that  on  B  is 
brought  to  250,  that  on  A  to  125,  and  the  bank  plugged 
in.  If  L  contains  50  volt  lamps  they  must  be  plugged 
3  in  series,  and  as  many  in  multiple  as  is  necessary  to 
put  on  the  load — keeping  A's  voltage  always  at  125.  If 
B  is  a  shunt  machine,  its  voltage  will  fall  as  the  load 
goes  on,  and  must  be  brought  up  by  cutting  resistance 
out  of  R'. 

The  situation  now  is  as  follows:  As  soon  as  L  is 
plugged  in,  B  sends  current  through  it,  and  through  A, 
.causing  A  to  run  as  a  motor.  The  high  resistance  of 
the  lamps  takes  up  the  125  volts  difference  between  the 
E.  M.  Fs.  of  A  and  B.  At  first  only  a  small  current 
flows,  but  it  is  gradually  increased  by  plugging  in  more 
lamps  in  multiple  till  the  full  load  is  reached.  Both  A 
and  B  are  now  loaded;  A  working  as  a  motor  and  assist- 
ing the  engine,  and  B  driving  A  and  expending  the 
balance  of  its  load  on  L.  If  A  and  B  are  shunt  machines, 
after  the  heat  test  is  run,  and  voltage,  current,  and  speed 
are  right,  the  drop  on  R  and  R  is  taken,  so  the  engineer 


THE    COMPOUND-WOUND    DYNAMO.  363 

may  know  if  sufficient  box  range  is  allowed  for  the 
difference  due  to  variation  in  summer  and  winter  tem- 
peratures. 

The  engine  as  a  loss  supplier  is  not  a  very  flexible 
means  of  speed  variation,  so  if  it  happens  that  there  are 
no  pulleys  to  give  the  proper  dynamo  speed,  or  that  on 
account  of  overload,  low  steam  pressure,  or  what  not, 
the  engine  speed  is  below  normal,  the  dynamo  E.  M.  F. 
will  have  to  be  approximated  as  followrs:  Let  us  assume 
that  the  proper  dynamo  speed  is  1,000  revolutions  per 
minute  (r.  p.  m. ),  and  that  its  rated  E.  M.  F.  is  250 
volts.  Then,  on  the  assumption  that  the  E.  M.  F.  varies 
directly  as  the  speed,  we  have, 

I.OOO 

=  4; 


250 

4  revolutions  per  volt;  /'.  e.,  if  250  volts  will  require  1,000 
revolutions,  i  volt  will  require  4;  or  expressed  differently, 
1/4  volt  per  revolution.  Let  the  actual  speed  be  800 — 
200  too  low:  for  a  fall  of  200  revolutions  we  would  have 
a  fall  of  200  x  1/4  =  50  volts;  so  that  the  dynamo  in 
this  case  would  be  tested  at  200  volts.  The  assumption 
upon  which  this,  practice  is  based  is  only  approximate, 
and  the  only  true  way  to  apply  a  correction  is  to  experi- 
mentally determine  what  difference  in  voltage  is  caused 
by  a  difference  of  one  revolution,  both  on  open  circuit, 
and  with  normal  current  flowing. 

During  the  heat  run,  when  no  data  are  being  taken,  as 
much  as  possible  of  the  load  should  be  put  onto  the 
motor,  because  the  larger  part  of  this  is  returned  to  the 
system,  while  that  absorbed  by  L  is  wasted.  In  order  to 
increase  the  motor's  share  of  the  load,  its  C.  E.  M.  F. 


364  TESTING    OF    DYNAMOS    AND    MOTORS. 

is  raised  by  strengthening  the  field.  The  immediate 
effect  of  strengthening  the  motor  field  is  to  reduce  the 
total  load  of  the  system,  but  to  give  a  larger  proportion 
to  the  motor.  The  current  then  can  be  restored  to  its 
proper  value  by  plugging  in  more  lamps  in  multiple. 
Should  the  capacity  of  L  be  so  limited  as  not  to  admit 
of  full  load  with  A's  voltage  at  125,  it  can  be  lowered, 
care  being  had  that  the  life  of  the  bank  is  not  endan- 
gered, because  as  A's  C.  E.  M.  F.  is  lowered  so  is  the 
amount  of  impressed  voltage  which  drops  across  it — but 
the  drop  across  L  increases.  The  reader  must  note  this 
point:  When  one  machine  is  running  directly  back  on 
another,  /.  e.,  without  the  intervention  of  a  lamp  bank  or 
other  resistance,  to  increase  the  load  on  the  motor,  its 
field  is  weakened,  whereas,  if  a  bank  intervenes  the  field 
must  be  strengthened.  In  the  first  case,  the  end  in  view 
is  to  increase  the  total  energy  of  the  circuit,  and  this  we 
do  by  lowering  the  motor's  C.  E.  M.  F.,  and  thereby 
decrease  the  effective  resistance  in  circuit.  Were  we  to 
strengthen  the  field  it  would  decrease  the  total  load. 
In  the  second  case,  the  end  in  view  is  not  to  increase  the 
total  load  of  the  circuit,  but  to  increase  it  in  one  part 
and  decrease  it  in  another,  and  this  we  do  by  raising  the 
effective  resistance  in  the  motor  part  of  the  circuit  and 
lowering  it  in  the  lamp  bank  by  plugging  in  more  lamps. 
In  the  first  case  there  is  increase  in  the  total  energy  of 
the  circuit,  because  its  resistance  is  decreased,  thereby 
increasing  the  current,  while  the  impressed  E.  M.  F.  is 
kept  the  same.  In  the  second  case  there  is  simply  a 
transference  of  energy  from  one  part  of  the  circuit  to 
another,  for  while  we  temporarily  decrease  the  total  load 
by  strengthening  the  motor  field,  the  circuit  resistance  is 


THE    COMPOUND-WOUND    DYNAMO. 


365 


restored  to  its  former  value  by  means  of  the  lamp  bank. 
This,  test  is  similar  somewhat  to  Test  I,  in  that  the  two 
machines  are  in  series  with  the  lamp  bank,  but  differs 
from  it  in  that  here  one  machine  is  a  motor  and  the 
other  a  generator. 

The  heat  test  on  B  done,  the  next  step  is  to  change 
over  and  test  A  as  a  generator.  It  would  be  impossible 
to  self-excite  B  and  reduce  its  voltage  sufficiently  to 
admit  of  the  machine's  being  run  as  a  motor  from  A, 
because  the  introduction  of  so  much  resistance  in  its 
field  circuit  would  cause  it  to  drop  its  field;  so  B 
is  separately  excited.  This  does  away  with  the  neces- 
sity of  the  bank,  and  the  test  becomes  the  same  as 
Test  II. 

TEST  IV.— Motor-Generator  Test,  Three  Machines. — The 
lamp  bank  of  the  above  test  can  be  very  profitably 
replaced  by  a  second  125  volt  machine,  which  has  the 
double  advantage  that  twice  as  much  energy  is  returned 
to  the  dynamo,  and  that  three  machines  may  be  tested  at 
once,  instead  of  two.  In 
Fig.  117  the  belt  is  not 
shown,  but  all  three  ma- 
chines are  belted  to  the 
same  engine.  (We  say 
this,  because  it  is  not  in- 
frequently the  practice  to 


FIG.  117. 


run  the  generator  from  one  engine  and  feed  its  current 
into  a  motor  or  motors  attached  to  another  engine  which 
needs  help  badly;  this  does  not  constitute  a  "  motor 
generator"  test,  because  there  is  no  work  saved,  since 
the  machines  do  not  circulate  the  current  between  them 
but  expend  it  in  doing  work  elsewhere.  In  a  "motor 


366  TESTING    OF    DYNAMOS    AND    MOTORS. 

generator  "  test,  we  understand  that  not  only  are  two  or 
more  machines  being  tested  from  the  output  of  one,  but 
this  with  an  expenditure  of  energy  which  is  but  a  fraction 
of  that  one's  output;  whereas  in  this  case,  the  entire 
output  of  the  generator  is  utilized  elsewhere.  The  prac- 
tice is  of  course  more  economical  than  running  the 
generator  on  a  lamp  bank,  because  the  energy  is  then 
wasted.)  In  Fig.  117,  A  and  A'  are  the  two  machines 
the  sum  of  whose  E.  M.  Fs.  equals  that  of  B.  F,  F't  F" 
and  R,  R ',  R"  are  their  fields  and  rheostats  respectively. 
K,  as  usual,  is  the  connecting  switch.  With  K  open,  A 
and  A'  are  gotten  in  series.  The  manner  of  doing  this 
seems  to  depend  on  the  man  that  is  doing  it.  One  man 
will  let  A  and  A  pick  up  on  their  residual  magnet- 
ism regardless  of  polarity,  and  if  necessary  reverse  the 
armature  cables  connecting  the  two  machines;  another 
man  will  let  one  machine  pick  up  and  charge  the  other 
from  it  by  raising  the  brushes,  closing  K,  and  the  head- 
board switches,  if  there  are  any,  and  sending  the  current 
around  through  B.  A  and  A  once  in  series  can  be 
regarded  as  a  single  250  volt  machine,  and  be  used  as 
such  to  charge  B'<=>  fields.  All  machines  excited,  and 
250  volts  on  both  sides  of  K,  the  voltmeter  should  read 
zero  across  it.  Closing  K,  the  fields  on  A  and  A  are 
gradually  weakened,  the  usual  attention  being  paid  to 
the  brushes.  By  having  a  voltmeter  across  each  machine, 
the  voltage  on  B  can  be  kept  up  to  250,  and  equally  dis- 
tributed between  A  and  A. 

TEST  V. — Motor  Generator  Test,  Machines  of  Different 
Current  Capacity. — The  problem  of  this  test  is  to  "run 
back  "  on  each  other,  two  machines  of  the  same  voltage, 
but  of  different  current  capacities.  The  test  of  the 


THE    COMPOUND-WOUND    DYNAMO.  367 

smaller  machine  is  the  same  as  Test  II,  for  there  is  no 
danger  in  the  smaller  machine's  maximum  current  injur- 
ing the  larger  machine.  But  in  testing  the  larger  machine 
as  a  generator  it  is  necessary  to 
have  an  auxiliary  motor  of  the 
same  voltage,  or  a  lamp  bank  to 
absorb  all  current  in  excess  of  the 
smaller  machine's  full  load.  Fig. 
118  gives  the  connections  when 
a  motor  is  used,  and  Fig.  119, 

those  of  a  lamp  bank.  A  and  B  are  the  machines  under 
test,  C  the  auxiliary  machine,  A  is  the  generator,  B  and 
C  the  motors.  To  simplify  the  diagram,  the  fields  and 
rheostats  are  not  shown.  K  is  B's  switch,  A",  C's.  A, 
£,  and  C'are  machines  of  the  same  voltage,  say,  125,  and 
are  belted  to  the  same  engine.  Call  A's  current  capac- 
ity 480  amperes;  ^'s,  320  amperes;  C's,  160  amperes. 
C's  capacity  can  be  anything  exceeding  the  difference 
between  A's  and  B's.  To  start  the  test,  a  field  is  gotten 
on  A.  l?s  and  C's  fields  are  then  charged  one  at  a  time, 
by  raising  their  brushes  and  closing  their  respective 
switches.  The  voltages  on  all  three  machines  are  then 
K.  adjusted  to  125  volts,  and  the  volt- 

1 — ^ 1      meter  should  read   zero  across  K 

eO  [U   and  K'.     When  this  is  the  case,  K 

1 — ( i — J     is  closed  and   B's   field    weakened 

FIG  no  until   B  carries   a  current   of   320 

amperes.     A's   voltage    meanwhile 

being  kept  at  125  volts  by  means  of  its  rheostat.  We 
now  read  across  K  to  insure  that  everything  is  right, 
and  if  it  is,  close  K':  next  weaken  C's  field;  it  will 
take  load,  and  unless  A's  voltage  is  kept  up  to  125 


368  TESTING    OF    DYNAMOS    AND    MOTORS. 

volts  B  will  lose  some  load.  So  it  is  best  to  keep 
up  A's  voltage,  and  gradually,  by  means  of  C's  brushes 
and  rheostat,  bring  its  load  up  to  160  amperes.  The 
currents  are  read  from  two  ammeters  preferably  placed 
as  indicated  in  the  diagram,  where  one  meter  is  in 
the  main  circuit  and  reads  A's  current,  while  the 
other  is  in  ^'s  circuit;  their  difference  gives  C's  cur- 
rent if  it  is  necessary  to  know  it.  If  neither  meter  has 
range  enough  for  the  main  circuit,  they  may  be  placed  in 
the  motor  circuits  where  their  combined  readings  give 
A's  current.  The  situation  of  the  meters  must  be  borne 
in  mind,  because  if  the  tester  labors  under  the  impression 
that  one  of  the  motor  meters  is  A's  meter,  both  A  and 
the  motor  will  be  heavily  overloaded.  In  putting  on  the 
load,  the  man  at  the  brushes  must  keep  them  at  the  non- 
sparking  point,  and  the  tester  at  the  field  boxes,  which 
are  all  placed  together,  must  regulate  the  load,  and  when 
the  ammeter  needles  cease  vibrating,  indicating  that  the 
brushes  are  at  rest,  he  can  adjust  the  load  exactly.  If 
the  motor  man  does  not  know  his  business,  he  is  likely  to 
make  trouble  by  either  moving  the  brushes  too  much 
at  once  or  by  moving  them  in  the  wrong  direction;  in 
the  latter  case  he  may  even  reverse  the  nature  of 
the  machines.  The  proper  way  to  handle  motor  brushes 
in  a  ''motor  generator"  test,  is  to  let  the  brushes  alone 
until  they  show  signs  of  sparking,  then  to  let  the  spark- 
ing keep  a  little  ahead  of  the  brush  movement.  The 
man  then  knows  where  he  is.  If  at  no  load  or  small  load 
any  of  the  machines  begin  to  spark  badly,  the  indications 
are  that  by  some  injudicious  movement  of  brushes  or 
rheostat  the  voltage  of  one  of  the  motors  has  been  raised 
above  that  of  the  dynamo,  resulting  in  a  reversal.  Un- 


THE    COMPOUND-WOUND    DYNAMO.  369 

der  tl;ese  circumstances  it  is  best  to  pull  the  switches  (A"- 
and  A' or  A"  and  A')  and  begin  again.  In  an  emergency 
the  load  can  be  thrown  off  by  opening  A'2,  but  one  of  the 
other  switches  should  be  opened  immediately  after  it,  as 
otherwise  B  and  C  run  back  on  each  other  with  possibly 
some  sparking  should  there  happen  to  be  much  of  a  dif- 
ference in  their  voltages.  In  this  case  the  engine  will 
keep  the  system  moving,  and  we  have  the  conditions  of 
Test  II.  The  smoothest  way  to  shut  down  the  whole 
system  is  to  shut  down  the  engine,  without  touching  the 
machines.  The  quickest  way  to  remove  one  of  the 
motors  from  service  is  to  pull  its  switch,  then  if  its 
rheostat  is  not  disturbed  it  can  be  replaced  in  service  by 
simply  closing  the  switch  again.  The  method  generally 
adopted  is  to  strengthen  the  field  on  the  motor  to  be  re- 
moved, until  its  load  has  fallen  nearly  to  zero,  then  to 
pull  its  switch.  The  removal  of  one  motor  will  slightly 
overload  the  remaining  one,  unless  A's  voltage  is  kept  at 
125  volts.  If  A's  voltage  is  not  kept  at  125  volts,  the  re- 
moval of  one  machine  will  influence  the  remaining  one 
only  so  far  as  A's  terminal  voltage  is  influenced  by  the 
removal  of  some  of  the  load.  It  amounts  practically 
to  the  same  as  pulling  one  motor  switch,  and  in  principle 
is  the  same  as  removing  any  consumer's  motor  from  a 
power  circuit,  except  that  in  this  case  there  being  but  one 
generator,  and  the  two  motors  constituting  its  entire  load, 
the  removal  of  one  of  them  makes  a  perceptible  differ- 
ence in  the  line  voltage.  If  A  is  a  perfectly  compounded 
machine,  tampering  with  the  load  of  one  of  the  motors 
or  even  pulling  the  motor  switch  will  not  affect  the  other 
motor,  because  the  self-regulation  of  the  compound- 
wound  generator  keeps  the  terminal  voltage  constant.  In 


370  TESTING    OF    DYNAMOS    AND    MOTORS. 

the  case  of  A's  being  compound-wound,  its  shunt  wind- 
ing is  adjusted  once  for  all  (hot),  to  give  normal  voltage 
on  open  circuit,  and  thereafter  the  series  winding  pre- 
serves constancy  of  voltage  throughout  the  range  of 
load. 

When  two  motors  run  in  multiple  from  one  dynamo,  the 
effective  resistance  in  circuit,  /.  e.,  opposition  to  current 
flow,  is  of  two  kinds:  (i)  Ohmic  resistance  of  armatures, 
leads,  cables,  etc.,  made  as  low  as  possible  and  negligible 
in  this  case;  (2)  C.  E.  M.  F.  of  the  motors,  and  this  is 
all  important.  The  current  flowing  may  be  expressed  by 
Ohm's  Law  as  follows: 

Imp.  E.  M.  F.  -  C.   E.   M.   F.  E  -  e 

J    -  r  _  Q»-     T   -     _ 

Ohmic  Resistance  R 

The  larger  e  is,  the  less  difference  is  there  between  E 
and  ^  hence  the  less  the  value  of  the  fraction 


R     > 

which  /  equals.     Therefore,  since  /  decreases  as  e  in- 
creases, /is  inversely  proportional  to  e. 

In  the  case  of  two  motors  in  multiple,  the  effective  re- 
sistance of  the  circuit,  neglecting  the  ohmic  resistance, 
is  the  multiple  "  resistance  "  of  the  two  C.  E.  M.  Fs. 
This  can  be  worked  out  ia  precisely  the  same  way  as 
multiple  ohmic  resistance  is.  The  "resistance"  of  one 
motor's  C.  E.  M.  F.  can  be  gotton  as  follows:  From  the 
formula 


R 


THE    COMPOUND- WOUND    DYNAMO.  37! 

from  which  we  get  e  =  E  —  /  R;  i.  e.,  the  C.  E.  M.  F.  is 
what  is  left  after  we  subtract  the  drop  through  resistance 
R  from  the  impressed  E.  M.  F.  Now  /<",  /,  and  R  can 
be  measured  with  voltmeter  and  ammeter,  whence  per- 
forming the  operations  indicated  by  the  formula,  we  get 
the  value  of  e  expressed  as  a  resistance.  Getting  thus  the 
value  of  e  on  both  motors  at  the  given  load  and  speed, 
we  combine  them  according  to  the  law  of  multiple  resist- 
ances and  get  the  effective  "  resistance "  of  the  two 
motors'  C.  E.  M.  Fs.  in  multiple. 

For  our  present  purpose  it  is  only  necessary  to  ob- 
serve that  lowering  the  C.  E.  M.  F.  of  either  or  both 
machines  will  increase  the  load,  and  that  the  greater  load 
will  go  on  the  motor  of  lower  C.  E.  M.  F. 

To  remove  the  load  we  can  weaken  the  dynamo  field 
instead  of  strengthening  the  motor  fields;  if,  however, 
A's  E.  M.  F.  be  at  any  time  brought  below  the  C.  E.  M. 
F.  of  B  and  C,  reversal  takes  place  with  the  usual  accom- 
paniment. The  demonstration  is  most  violent  when  a 
field  is  broken  on  some  machine,  thus  depriving  it  of  all 
ability  to  oppose  an  inflow  of  current  from  the  other 
machines.  If  A's  field  gets  broken,  its  E.  M.  F.  falls 
nearly  to  zero:  B  and  C  become  generators,  and  short 
circuit  through  it;  if  it  is  compound-wound,  it  will  try  to 
reverse  its  direction  of  rotation.  If  B  and  Care  shunt- 
wound,  they  will  drop  their  fields,  but  not  before  they  lose 
their  belts.  If  compound-wound,  the  differential  action 
of  their  fields  (cumulatively  connected  to  run  as  motors) 
puts  a  limit  to  their  current.  Should  we  break  a  field  on 
one  of  the  motors,  say  B,  A  and  C,  as  generators,  would 
short  circuit  through  it,  and  the  very  characteristic  howl 
which  rends  the  air  when  large  units  are  involved,  once 


372  TESTING    OF    DYNAMOS    AND    MOTORS. 

heard,  is  never  forgotten.  This  trouble  with  reversals 
is  not  confined  to  testing  rooms,  but  is  liable  to  occur 
wherever  dynamos  are  run  in  multiple.  All  field  connec- 
tions should  therefore  be  inspected  at  regular  intervals, 
for  connectors  or  box  wires  are  liable  to  rust  off  or  jar 
loose. 

In  the  test  above,  as  soon  as  the  test  on  A  as  a  genera- 
tor is  completed,  C's  switch  can  be  pulled,  A's  voltage 
gradually  reduced,  B's  increased,  and  when  the  current 
is  nearly  zero,  the  motor  man  signaled  that  reversal  is 
about  to  take  place,  so  that  he  can  bring  ^'s  brushes 
backward  and  A's  forward.  The  field  on  A  is  then  weak- 
ened, and  A  takes  a  load  as  a  motor.  This  change-over 
without  shutting  down  we  have  learned  to  be  practicable 
only  when  shunt  or  separately  excited  machines  are  in- 
volved. If  any  of  the  machines  are  compound-wound,  the 
system  must  be  shut  down  and  the  series  fields  reversed. 
The  reasons  are  these:  If  on  a  shunt  machine,  running 
as  a  dynamo,  we  assume  a  certain  relation  to  exist 
between  the  directions  of  current  flow  in  field  and  arma- 
ture, this  relation  is  changed  upon  the  machine  becom- 
ing a  motor.  On  a  series  motor  the  relation  remains  the 
same.  In  other  words,  a  compound-wound  machine 
cumulatively  connected  and  running  as  a  motor,  will,  if 
its  C.  E.  M.  F.  is  allowed  to  exceed  the  impressed 
E.  M.  F.  (that  is  to  say,  to  become  a  generator,  as  B 
•does  when  its  C.  E.  M.  F.  is  made  to  exceed  the  E.  M.  F. 
of  A,),  have  its  series  field  current  reversed,  though 
the  shunt  field  remains  the  same  as  before.  The  two 
fields  are  then  opposed,  and  it  will  be  impossible  to  work 
on  a  load,  because  the  windings  neutralize  each  other. 

If  at  the  signal  for  reversal  the  motor  man  should  rock 


THE    COMPOUND-WOUND    DYNAMO.  373 

his  brushes  the  wrong  way,  the  load  will  seem  to  hesi- 
tate for  an  instant,  and  then  will  go  on  with  a  rush; 
how  much  of  a  rush  depending  upon  the  position  of 
the  brushes  and  the  type  of  the  machines.  If  A,  B,  and 
C  are  shunt-wouno^  machines,  the  result  will  be  to  put 
on  some  load  in  the  original  way,  /.  e.y  before  it  was 
attempted  to  make  A  the  motor.  If  A,  B,  and  C  are 
compound. wound,  the  conditions  will  not  be  the  same, 
because  their  series  fields  having  been  reversed,  their 
ability  to  generate  in  the  original  way  is  limited;  the 
result  is  a  large  initial  flow  of  current,  which  is,  how- 
ever, soon  checked  by  the  differential  action  of  A's  field. 
More  or  less  sparking  attends  these  reversals,  its  gravity 
depending  upon  the  position  of  the  brushes.  If  either 
B  or  C  is  a  compound-wound  machine  and  A  is  a  shunt 
machine,  and  some  careless  move  makes  the  compound- 
wound  machine  a  motor  when  it  is  connected  up  as  a 
generator,  the  trouble  will  be  more  serious,  for  as  soon  as 
current  from  A  flows  through  the  compound-wound 
machine,  say  B,  B's  series  field  neutralizes  the  shunt 
field,  and  a  short-circuit  follows.  If  B  is  separately  ex- 
cited matters  are  even  worse,  for  the  machine  cannot  drop 
its  field,  whereas  shunt  machines  sometimes  do  so  before 
the  belt  can  fly  off.  Sometimes  the  compound-wound 
machine  will  throw  its  belt  trying  to  reverse  rotation, 
showing  that  not  only  has  the  series  winding  neutralized 
the  shunt  winding,  but  overpowered  it,  thereby  revers- 
ing H's  polarity. 

TEST  VI. — Same  as  Test  V.,  with  Lamp  jBan^.—This  test 
is  practically  the  same  as  Test  V.,  excepting  that  a  lamp 
bank  replaces  the  auxiliary  machine  C.  Fig.  120  gives  the 
connections.  A  is  the  125  volt,  480  ampere  machine  to  be 


374  TESTING    OF    DYNAMOS    AND    MOTORS. 

run  as  dynamo ;  B,  the  1 25  volt,  320  ampere  machine  to  be 
run  as  motor;   and  Z,  a  125  volt  lamp  bank  capable   of 
carrying    160  amperes;  L  and  B  are  in  multiple,  and   A 
divides  its  load  between  them.     In 
plugging   in   the    lamps  the  motor 
must  be  watched  and  its  load  regu- 
lated with  its  field  rheostat.     The 
general  precautions  to  be  observed 
are  similar  to  those  of  Test  V. 
JTIG  I2O  TEST    VII. — Same    as     Test    V., 

with    Motor. — This     test,     a    very 

natural  outcome  of  Test  V.,  is  used  also  where  two 
dynamos  of  the  same  E.  M.  F.,  but  of  different  current 
capacities,  are  to  be  tested.  A  third  machine,  C,  is  used 
as  motor  to  absorb  the  current  from  dynamos  A  and  B. 
The  connections  are  the  same  as  those  of  Fig.  118,  and 
the  test  is  almost  the  same.  The  exception  being  that 
when  the  switches  are  closed  A's  field  is  weakened,  and 
not  those  of  B  and  C.  A  is  now  the  auxiliary  machine. 
If  either  B  or  C,  or  both,  are  compound-wound,  care 
must  be  had  .that  the  fields  are  cumulatively  connected 
as  dynamos,  while  A  must  be  cumulatively  connected  as- 
motor. 

TEST  VIII.—  Machines  of  Different  E.  M.  F.  and  Current 
Capacity. — This  is  a  test  that  the  writers  have  never  been 
called  upon  to  run,  but  which  is  perfectly  feasible 
should  occasion  demand  it.  Fig.  121  shows  the  connec- 
tions. A  and  B  are  machines  of  widely  varying  E.  M.  Fs., 
also  current  carrying  capacities,  and  two  banks,  L  and  Z', 
are  auxiliaries  to  the  test;  A  is  a  250  volt,  480  ampere 
machine,  B  is  a  125  volt,  300  ampere  machine,  Z  is  a 
bank  in  series  with  B  to  absorb  A's  excess  of  voltage. 


THE    COMPOUND-WOUND    DYNAMO.  375 

and  Z',  a  bank  in  multiple  with  A  to  take  its  excess  of 
current.  A,  B,  and  L  are  first  put  on  as  in  Test  III. 
The  system  then  stands  at  the  proper  voltage,  but  with 
only  the  current  at  A's  maximum  capacity 
of  300  amperes.  The  remaining  180  am- 
peres are  put  on  L'  as  in  Test  VI. 

TEST  IX. — Single  Machine. — Where 
there  is  only  one  machine  to  be  tested, 
or  where  machines  are  run  from  engines 
between  which  no  connections  can  be 
made,  the  motor-generator  test  is  not 
practicable,  and  compounding  must  be 
done  on  a  lamp  bank  or  water  box.  The  cost  of  banks 
of  such  capacity  as  some  machines  would  require  pre- 
cludes their  use,  and  water  boxes  are  found  to  be  a 
very  cheap,  simple,  and  satisfactory  substitute.  Water 
box  compounding  is  simpler  than  the  motor-generator 
test,  in  that  it  reduces  the  number  of  difficulties  possible 
to  be  encountered,  but  is  not  nearly  so  economical.  In 
the  motor-generator  test,  the  energy  supplied  to  the  sys- 
tem is  merely  that  necessary  to  overcome  the  mechanical, 
electrical,  and  magnetic  losses  in  the  two  machines, 
whereas  a  water  box  or  other  ohmic  resistance  dissipates 
the  entire  output  of  the  machine  under  test.  But  where 
there  is  but  a  single  machine  there  is  no  way  of  return- 
ing its  energy  to  the  system,  so  that  the  waste  is  an 
unavoidable  necessity.  A  box  4x5x8  feet  will  carry 
the  full  load  of  a  550  volt,  500  KW  generator.  Its  iron 
plates,  one  of  which  is  movable,  the  other  stationary, 
should  have  as  much  cross-section  as  the  size  of  the  box 
will  allow,  and  flexible  cables  must  be  attached  to  these 
plates  as  permanent  leads.  When  a  box  is  used  for  the 


TESTING    OF    DYNAMOS    AND    MOTORS. 

first  time,  it  is  well  to  be  on  the  safe  side,  and  pull  the 
plates  as  far  apart  as  possible,  and  fill  it  only  half  full 
of  water.  As  a  further  precaution  the  voltage  on  the 
dynamo  can  be  reduced  until  we  ascertain  what  the  box 
will  do.  The  box  terminals  are  connected  through  a 
switch,  K,  to  the  dynamo  terminals  as  in  Fig.  122,  where 
A  is  the  dynamo,  F,  its  field,  R,  its  rheostat,  and  Z,  the 
water  box.  K  is  a  long  break,  double-pole  spring  switch. 
Having  experimentally  found  out 
just  about  what  to  expect  of  Z,  A's 
E.  M.  F.  is  adjusted  to  500,  and 
K  closed.  The  load  can  then  be 
FIG.  122.  worked  on  by  putting  a  little  salt 

between  the  plates  to  improve  the 

conductivity  of  the  solution,  by  putting  in  more  water,  or 
pushing  the  plates  nearer  together,  any  of  which  proced- 
ures decreases  the  resistance  of  the  water  column.  As 
soon  as  current  begins  to  flow  the  series  windings  begin  to 
act:  this  raises  the  terminal  E.  M.  F.,  which  in  turn  aug- 
ments the  shunt  field.  The  field  rheostat  once  adjusted 
must  not  be  changed  throughout  the  test.  Care  must  be 
exercised  in  using  salt,  when  the  plates  are  near  together, 
as  a  handful  may  precipitate  a  heavy  overload.  Sal- 
ammoniac  is  sometimes  used  instead  of  ordinary  salt, 
but  the  writers  object  to  it  because  its  effect  is  much 
greater  than  that  of  salt,  but  is  not  permanent.  It  is 
admirably  adapted  to  temporarily  lower  the  water  box 
resistance  when  it  is  desired  to  make  a  series  machine 
pick  up  a  field,  but  ordinarily  it  will,  in  inexperienced 
hands,  cause  trouble.  In  any  case,  as  the  water  heats, 
its  resistance  decreases,  and  the  current  increases.  As  a 
factor  of  safety,  it  is  well  to  have  a  faucet  for  drawing 


THE    COMPOUN7D-WOUND    DYNAMO.  377 

off  some  of  the  water  should  the  current,  with  the  plates 
as  far  apart  as  possible,  still  be  too  high:  the  effect  of 
this  is  to  decrease  the  cross-section  of  the  water  column, 
and  thereby  to  increase  its  resistance.  One  point  easily 
overlooked,  but  which  should  be  borne  in  mind,  is  this: 
if  water  from  the  box  is  allowed  to  run  onto  the  ground, 
it  will  cause  a  short  circuit  if  there  happens  to  be  a 
ground  elsewhere  on  the  system.  For  this  reason  the 
water  should  be  run  into  a  wooden  or  paper  bucket,  and 
then  emptied  from  that.  So,  also,  if  decrease  in  load 
requires  the  addition  of  more  water,  it  must  not  be  made 
with  a  metal  bucket,  as  on  high  voltage  machines  such 
carelessness  is  fraught  with  danger.  At  full  load  the 
amount  of  water,  the  amount  of  salt,  and  the  distance 
between  the  plates  should  be  so  adjusted  that  all  load 
regulations  can  be  effected  by  means  of  the  movable 
plate.  A  little  thought  facilitates  this. 

With  full  current  load  on  it  will  probably  be  found  that 
the  voltage  is  too  high:  it  is  not  absolutely  necessary, 
but  if  desirable,  the  voltage  can  be  reduced  by  readjust- 
ing the  series  field  shunt;  or  better  still,  the  German 
silver  shunt  can  be  put  on  and  temporarily  adjusted.  Its 
permanent  adjustment  is  not  made  until  the  machine  has 
fully  heated  after  several  hours'  run,  when  the  load  is  re- 
moved, the  shunt  field  rheostat  readjusted  to  give  500  volts, 
and  the  load  again  put  on.  The  process  of  compounding 
is  next  in  order.  A  very  complete  and  detailed  account  of 
the  compounding  test  is  given  in  Test  X,  which  has  been 
especially  selected  with  the  view  of  giving  the  reader  a  good 
idea  of  what  is  to  be  watched  in  a  test  involving  many 
machines  of  different  degrees  of  compounding  and  widely 
varying  outputs.  Tests  X  and  XI  are  typical  tests. 


CHAPTER  XI. 

COMPOUNDING. 

TEST  X.  —  To  Run  Under  Full  Load,  Two  500  Volt, 
500  KW  Street  Railway  Generators,  and  to  Supply  the 
Loss  from  an  Auxiliary  Dynamo  of  the  Same  Voltage. — The 
conditions  selected  are  comprehensive  and  include  lia- 
bility to  many  peculiar  difficulties. 

The  main  factors  are  the  two  machines  under  test,  and 
the  loss  supplier  as  shown  in  Fig.  123.  A  and  B  are 


FIG.  123. 

belted  or  clutched  together,  and  C,  the  supplier,  is 
belted  to  an  engine.  In  the  case  where  the  loss  is 
supplied  from  an  engine  belted  to  both  machines,  the 
dynamo  is  driven  partly  by  the  engine  and  partly  by 
the  multipolar  motor.  In  the  present  case  the  dynamo 
is  driven  by  the  motor  alone.  In  the  first  case  the 
current  in  both  multipolar  machines  is  the  same;  in 
the  last  case  the  motor  current  exceeds  that  of  the 
dynamo  by  an  amount  equal  fro  what  flows  through  the 
supplier.  Where  C  is  an  engine,  the  speed  of  the  system 


COMPOUNDING.  379 

depends  upon  the  speed  of  the  engine  and  the  relative 
size  of  the  pulleys  used,  and  it  is  not  always  practicable 
to  have  pulleys  of  such  size  as  to  give  just  the  speed  at 
which  the  machine  compounds.  By  supplying  the  loss 
electrically,  almost  perfect  control  of  the  speed  is  secured 
by  varying  the  E.  M.  F.  of  the  supplier  by  means  of  its 
field  rheostat.  Here  again  the  great  flexibility  of  the 
electrical  system  of  power  supply  is  illustrated,  and  what 
is  true  here  is  true  wherever  electricity  competes  with 
other  sources  of  motive  power. 

In  cases  where  the  engine  driving  the  loss  supplier  is 
small  or  doing  other  work,  it  is  customary  to  have  C 
consist  of  two  dynamos  in  series,  so  that  should  the 
engine  speed  for  any  reason  fall  off,  the  required  E.  M.  F. 
can  still  be  maintained  by  field  regulation  on  C.  We  will 
suppose,  then,  that  C  consists  of  a  500  volt,  100  kilowatt 
dynamo  in  series  with  a  125  volt,  60  kilowatt  dynamo. 
To  get  the  two  machines  in  series,  a  voltmeter  is  required. 
With  both  fields  excited,  and  any  two  brushes  or  terminals 
of  the  respective  machines  joined  together,  the  voltmeter 
should  read,  across  the  unconnected  terminals,  the  sum 
of  the  voltages  of  each  machine.  Should  it  read  their 
difference  it  proves  them  to  be  opposed,  and  one 
machine's  terminals  must  be  reversed,  or  its  field  polarity 
changed.  The  latter  method  is  generally  pursued,  and 
to  facilitate  the  change  the  low  volt  machine  is  sepa- 
rately excited,  a  feature  whose  other  advantages  will 
be  discussed  later.  The  loss  supplier,  C,  may  now 
be  connected  to  the  multipolar  motor,  B,  by  means 
of  cables.  If  the  cable  connections  are  made  first, 
and  the  two  suppliers  gotten  in  series  afterward,  the 
procedure  is  a  little  different.  With  all  brushes  down 


380  TESTING    OF    DYNAMOS    AND    MOTORS. 

and  all  switches  except  one  closed,  the  voltmeter  is 
placed  across  the  open  switch:  should  the  reading  be 
lower  than  that  on  one  machine  alone,  or  zero,  the  indi- 
cation is  that  in  the  first  case  the  voltages  are  opposed; 
in  the  second  case  they  are  opposed  and  equal,  or  there 
is  an  open  circuit.  An  open  circuit  can  be  due  among 
other  things  to  failure  to  press  the  meter  push  button, 
some  open  switch,  raised  brushes,  or  incomplete  connec- 
tion overlooked,  shellac  on  a  commutator,  lacquer  on 
the  points  where  the  voltlines  are  applied,  or  as  the  case 
often  is,  the  points  of  the  voltlines  are  so  oxidized,  from 
other  uses,  as  to  be  non-conducting.  The  machines  once 
in  series,  and  the  cable  connections  made,  it  is  wise  to 
reduce  C's  voltage,  and  to  put  a  field  on  the  motor,  B, 
as  a  safeguard  in  case  a  switch  should  fall  to  or  be  closed 
by  mistake. 

During  the  writers'  earlier  experience  it  was  customary 
to  connect  the  motor  shunt  windings  in  series  multiple  in 
motor-generator  tests  of  large  machines,  and  to  sepa- 
rately excite  them  to  their  full  voltage — a  practice  based 
upon  the  following  reasons:  i.  The  magnetizing  power 
of  the  shunt  winding  is  much  increased,  the  iron  of  the 
fields  is  brought  almost  to  saturation  by  these  windings 
alone,  thus  minimizing  the  effect  of  the  series  coils  at 
starting,  and  lessening  the  liability  to  phenomena  here- 
after to  be  described.  2.  With  the  shunt  fields  in 
multiple  the  load  can  be  worked  on  more  gradually,  for 
the  start  is  made  with  saturated  fields,  and  as  the  shunt 
field  current  is  weakened  the  iron  becomes  less  saturated 
and  the  series  coils  begin  to  take  firmer  hold,  but  at  a 
rate  easily  controlled.  3.  The  strong  field  admits  of  an 
easy  start  without  an  excessive  current.  Altogether 


COMPOUNDING.  381 

then,  connecting  half  the  spools  in  multiple  simplifies 
starting.  In  inexperienced  hands  the  multiple  connec- 
tion had  best  be  adopted,  at  least  to  begin  with. 

On  general  principles  and  for  the  following  reasons 
it  is  now  the  custom  to  connect  in  series  all  fields  intended 
to  run  in  series  under  actual  working  conditions:  i.  On 
machines  of  four  or  more  fields,  designed  to  be  connected 
in  series,  it  is  unwise  to  run  them  in  series  multiple,  be- 
cause unless  great  care  is  taken  they  are  liable  to  be 
injured  by  overheating.  2.  Should  a  coil  be  defective, 
it  has  three  or  more  good  ones  in  series  with  it  acting  as  a 
factor  of  safety.  3.  The  series  connection  economizes  in 
the  number  of  field  rheostats  required  to  run  the  test,  and 
simplifies  the  cutting  out  of  a  defective  rheostat  without 
shutting  down  the  test:  because,  with  fields  in  multiple, 
the  applied  E.  M.  F.  remaining  the  same,  the  total  field 
current  is  four  times  as  great  and  the  current  capacity 
of  the  rheostat  must  be  increased  accordingly  by  placing 
more  boxes  in  multiple;  but  as  this  decreases  their  resist- 
ance, and  hence  the  scope  of  variation,  it  is  necessary  to 
put  more  boxes  also  in  series.  To  cut  out  one  of  a  num- 
ber of  boxes  in  series,  it  is  only  necessary  to  cut  out  its 
resistance  and  cut  in  the  same  amount  on  another  box, 
to  keep  the  field  current  constant,  and  then  "jump"  a 
piece  of  wire  across  the  terminals  of  the  defective  box. 
In  connecting  boxes  in  series  multiple  it  is  better  to 
adopt  the  plan  of  Fig.  124  than  that  of  Fig.  125,  because 
in  the  latter  case  any  fault  with  one  box  affects  to  a 
greater  degree  every  other  box  than  in  the  former  case, 
and  in  case  of  open  circuit  in  one,  every  one  with  which 
it  is  in  series  becomes  useless,  and  throws  the  entire  field 
current  on  the  remaining  series  set.  4.  With  a  given 


382  TESTING    OF    DYNAMOS    AND    MOTORS. 

current  in  the  armature  the  motor  brushes  do  not  require 
to  be  brought  as  far  back  with  the  comparatively  weak 
field  as  with  a  strong  one,  and  in  putting  on  a  load  this 
is  a  desirable  feature.  In  such  a  test  the  load  is  put 
on  as  in  other  motor-generator  tests,  by  lowering  the 

C.  E.  M.  F.  of  the  motor.  This  is 

_T^  I  ^ lowered  by  either  weakening  the  field 

L-Q— HI!— LQ— '  current,  or  by  giving  the  brushes  a 
FIG.  124.  negative  lead,  so  as  to  bring  the  poles 

of  the  armature  in  a  position  to 
neutralize  some  of  the  field  magnetism.  In  the  fol- 
lowing test  we  will  assume  the  fields  to  be  four 
in  number  and  connected  in  series.  It  is  a  practice 
gaining  hold  to  run  the  armatures  of  multipolar  carbon 
brush  dynamos  against  the  brushes,  so  that  in  starting  a 
test  the  direction  of  rotation  is  at  least  fixed,  and  the 
relation  between  the  field  and  armature  polarities  must 
be  adjusted  accordingly.  Now  since  both  series  and 
shunt  windings  are  on  the  spool,  the  current  must 
pass  around  both  windings  in  the  same  direction,  and  to 
insure  that  this  shall  be  the  case,  it  must  be  tested  out. 
The  test  consists  in  taking  the  drop  first  across  one 
winding,  and  then  the  other  on  any  given  spool.  The 
deflections  should  be  in  the  same  direction  in  both  cases. 
The  series  field  resistance  being  very  low  and  having  only 
that  current  necessary  to  turn  the  motor  over  slowly, 
the  drop  across  it  will  be  only  a  fraction  of  a  volt,  and 
the  instrument  must  be  delicate  to  indicate  it.  On  the 
other  hand,  as  the  drop  on  the  shunt  spool  is  consider- 
able, the  galvanometer  must  be  shunted  or  otherwise 
protected  before  being  submitted  to  this  drop.  Since  it 
is  merely  the  direction  and  not  magnitude  of  deflection 


COMPOUNDING.  383 

sought,  it  suffices  to  shunt  the  galvanometer  terminals 
with,  a  piece  of  copper  wire. 

In  connecting  the  multipolar  dynamo  fields  the  test  of 
correct  polarity  is  made  on  the   shunt  field  as  follows: 
Attach  the  two  shunt  field  terminals 
to   the  blocks  on    either  side  of  the 
machine,  as  in  Fig.    126,    and    insert 
the    field    rheostat    wherever     it    is  FIG.  125. 

most  convenient  to  do  so;  next 
remove  one  of  the  brush  holder  cables,  or  draw  the 
brushes  in  adjacent  holders  and  close  K,  Fig.  123,  to 
charge  A's  field  from  C.  A's  field  circuit  contains  a  low 
reading  ammeter  to  be  used  in  finding  the  field  and 
rheostat  resistances,  and  to  serve  as  a  check  on  the  cur- 
rent necessary  to  give  the  required  ampere-turns  for 
normal  voltage.  Upon  closing  K,  the  ammeter  needle 
must  be  observed,  to  see  if  it  moves  at  all,  or  in  the  right 
direction.  In  lieu  of  an  ammeter,  a  slight  spark  upon 
opening  K  indicates  the  field  circuit  to  be  closed.  As 
soon  as  the  field  is  charged,  open  K  and  replace  the 
brush  holder  cable,  and  A  should  generate  its  own  field, 
if  its  connections  are  proper.  If  it 
/~  ~\  refuses  to  generate,  its  shunt  field 

/     /         \     \      connections  must  be  reversed. 
'  ^ 


reason    f°r   charging  from   C 
is  considered  elsewhere.     The  proper 
FIG.  126.  connections  made  on  A,  B,  and    C, 

.Z?'s  field  excited,  brushes  all  down, 
and,  as  a  last  precaution,  all  connections  inspected,  the 
test  is  ready  to  start.  There  are  two  methods  of  start- 
ing: First,  to  use  a  lamp  bank  as  a  starting  box  in  series 
with  the  motor  armature,  bringing  up  the  speed  by  grad- 


384  TESTING    OF    DYNAMOS    AND    MOTORS. 

ually  cutting  in  the  bank  and  finally  short-circuiting  the 
bank  when  £'$  C.  E.  M.  F.  is  well  up;  second,  to  reduce 
C's  E.  M.  F.,  close  K\  and  start  the  system  on  low  volt- 
age. The  E.  M.  F.  is  then  gradually  raised  till  full  speed 
is  attained. 

The  former  of  these  methods  would  seem  to  be  pref- 
erable, as  there  is  little  likelihood  of  brush  and  belt 
troubles,  and  C  is  not  called  upon  to  generate  excessive 
current. 

The  second  method  requires  some  care  to  avoid  com- 
plications when  heavy  machines  are  involved.  If  C  be  a 
shunt  machine  it  must  be  separately 
excited,  as  otherwise  it  will  lose  its 
field  when  it  is  attempted  to  lower  its 
E.  M.  F.  for  starting  B.  If  shunt- 
wound,  and  used  in  conjunction  with 
a  lamp  bank,  .Z?'s  field  must  be  con- 
nected beyond  the  lamp  bank,  as  in 
Fig.  127,  so  that  there  will  be  the  strongest  possible  field 
even  when  the  bank  is  in  circuit.  In  starting  with  a  bank, 
the  number  of  lamps  to  be  plugged  in  series  depends  upon 
the  E.  M.  F.  used,  while  the  number  to  be  plugged  in  mul- 
tiple depends  upon  the  current  necessary  to  start  the 
system.  When^"',  Fig.  123,  is  first  closed,  the  total  drop 
is  through  the  lamps,  as  the  motor  before  starting  has 
no  C.  E.  M.  F. ;  as  the  motor  begins  to  move,  and 
increases  in  speed,  its  C.  E.  M.  F.  rises  and  absorbs  part 
of  the  impressed  E.  M.  F.,  when  the  drop  across  the 
lamps  becoming  less,  they  grow  dimmer.  The  bank  can 
next  be  replugged  with  fewer  lamps  in  series,  and  more 
in  multiple,  and  when  the  glow  becomes  almost  imper- 
ceptible the  bank  is  short  circuited  with  a  plug,  because 


COMPOUNDING.  385 

at  this  stage  £'s  C.  E.  M.  F.  is  sufficient  to  control  the 
current  value.  In  replugging  a  bank  care  must  be  taken 
that  the  speed  is  each  time  given  a  good  chance  to 
respond  to  the  change  in  the  bank,  otherwise  the  lamps 
may  be  submitted  to  an  unsafe  voltage. 

The  second  or  "  low  volt  "  method  of  starting  dispenses 
with  the  use  of  a' sometimes  complicated  and  expensive 
bank,  and  in  experienced  hands  need  give  no  trouble. 
As  a  time  saver  it  is  particularly  valuable  when  local 
troubles  with  belts,  brushes,  and  bearings  necessitate 
shutting  down  frequently.  In  general,  the  writers' 
experience  has  been  that  a  5oo-volt  compound-wound 
multipolar  street  railway  generator  cumulatively  con- 
nected as  motor  will  start  smoothly  under  a  pressure  of 
25,  35,  45,  50,  or  60  volts,  according  as  the  machine  is  a 
100,  200,  300,  400,  or  500  kilowatt  machine.  These  figures 
are  averages.  If  with  75  volts  across  C,  B  fails  to  start 
upon  closing  A",  the  wiring  must  be  inspected  to  see  that 
no  error  has  been  made.  If  there  is  no  spark  upon  open- 
ing K\  it  indicates  an  open  circuit,  which  may  be  due'  to 
an  open  switch,  raised  brushes,  an  absent  fuse,  or  a  loose 
connection.  If  C  consists  of  two  dynamos  in  series,  it  is 
convenient  to  have  the  one  of  higher  voltage  compound- 
wound,  and  its  mate  separately  excited,  not  only  to  facili- 
tate reversing  its  polarity  if  necessary,  but  to  render  it 
active  or  inert  according  as  its  E.  M.  F.  is  needed  or 
not;  besides  this,  separate  excitation  makes  certain  that 
the  field  will  pick  up  when  it  is  needed.  The  separately 
excited  machine  should  be  of  large  current  capacity  and 
low  voltage,  as  compared  with  its  mate,  for  on  starting 
it  has  a  large  armature  current,  and  no  field  to  determine 
the  neutral  point.  The  result  is  that  the  pole  heads  are 


386  TESTING    OF    DYNAMOS    AND    MOTORS. 

magnetized  by  induction,  and  locate  the  neutral  line  in 
a  position  that  the  brushes  cannot  be  made  to  occupy. 
The  result  is  sparking  at  the  brushes,  an  effect  much 
aggravated  if  the  current  at  starting  exceeds  the  rated 
output  of  the  machine. 

Since  low  voltage  machines  carry  a  larger  current  than 
high  voltage  machines  of  the  same  output,  the  above 
precaution  lessens  the  danger  of  overload,  and  the 
brushes  will  probably  need  little  or  no  attention  through- 
out the  test. 

Failure  in  getting  the  system  started  upon  closing  the 
switch  may  be  due  to  the  total  absence  of  a  motor  field 
or  to  its  weakness,  a  condition  to  be  detected  primarily 
by  holding  a  nail  or  iron  key  to  the  motor  pole  pieces 
before  closing  K'.  If,  however,  this  precaution  has  been 
neglected,  the  indications  are  apt  to  be  more  violent,  a 
belt  may  fly  off  and  the  supplier  brushes  spark  badly  if 
the  switch  is  not  promptly  opened.  As  such  cases,  or 
similar  ones,  arise  from  time  to  time,  it  is  good  practice 
to  use  carbon  brushes  on  all  machines  of  sufficient  volt- 
age to  admit  of  a  brush  of  suitable  cross-section.  "Of 
sufficient  voltage  "  is  specified,  because  on  a  machine  of 
large  output  and  low  voltage  the  current  would  be  so 
large  that  it  would  be  impracticable  to  use  carbon  brushes 
large  enough  to  carry  it.  Copper  brushes  under  certain 
circumstances  either  melt  and  run  on  to  the  commutator, 
or  the  component  wires  fuse  together,  rendering  the 
brush  unfit  for  further  use.  Carbon  brushes  are  used  on 
machines  of  250  volts  and  over,  and  are  especially  adapted 
for  use  on  machines  subjected  to  wide  and  rapid  variations 
of  load,  as  in  street  railway  service. 

The  resistance  of  a  carbon  brush    is  high  enough  to 


COMPOUNDING.  387 

prevent  the  low  voltage  of  each  short  circuited  coil  pass- 
ing under  it  from  generating  a  large  current,  and  thereby 
causing  excessive  sparking  when  the  brushes  are  not  on 
the  neutral  line.  On  the  other  hand,  on  small  machines  of 
low  voltage  and  large  current  output,  the  resistance  of 
carbon  brushes,  practicable  to  use,  would  cause  so  great 
an  /  R  loss  as  to  be  very  uneconomical. 

For  starting  the  smaller  machines  the  voltage  due  to 
the  residual  field  of  the  loss  supplier  often  suffices :  this 
residual  field-can  be  increased  by  working  up  to  full  field 
strength,  and  then  gradually  breaking  the  field  by  means 
of  the  boxes.  Where  a  separately  excited  machine  is  the 
loss  supplier,  the  exciter's  field  may  be  broken  and  its 
own  left  intact,  thus  giving  the  residual  field  of  the 
supplier  the  additional  magnetism  generated  by  the 
small  current  which  the  residual  field  of  the  exciter  urges 
around  the  supplier's  field  coils. 

To  start  a  pair  of  multipolar  machines  the  field  of  one 
loss  supplier  is  opened:  the  line  switch  of  the  other 
is  the  only  break  in  the  motor  circuit,  and  the  voltage 
across  it  should  read,  25,  35,  45,  or  60  volts,  according 
as  the  machines  are  of  100,  200,  300,  or  500  kilo- 
watts' capacity.  Care  should  be  taken  that  the  voltlines 
make  good  contact,  as  otherwise  the  true  voltage  of  the 
machine  will  be  higher  than  that  indicated,  because  the 
true  voltage  will  be  partly  diverted  by  the  resistance  of 
the  poor  contact.  As  a  final  precaution,  before  closing 
the  switch  see  that  no  person  is  where  he  can  be  caught 
by  belt  or  pulley.  A  man  should  be  stationed  at  the 
motor  to  signal  if  the  direction  of  rotation  is  right. 
Since  the  motor  shunt  field  is  separately  excited,  if  the 
system  has  the  wrong  rotation,  it  can  be  righted  by- 


388  TESTING    OF    DYNAMOS    AND    MOTORS. 

reversing  the  shunt  field,  for  this  being  generally  much 
stronger  than  the  series  field,  dictates  the  direction  of 
rotation  even  though  the  latter  may  oppose  it. 

To  determine  if  the  two  windings  assist  each  other, 
any  of  several  methods  can  be  pursued.  One  method  is 
to  break  the  shunt  field  while  the  motor  is  turning  over 
slowly:  if  the  fields  are  properly  connected,  the  speed 
will  rise;  if  connections  are  such  as  to  oppose  shunt  and 
series  windings,  the  motor  will  stop  and  perhaps  start  up 
in  the  reverse  direction,  because,  if  the  fields  are  opposed 
they  tend  to  turn  the  armature  in  opposite  directions, 
but  the  weaker  series  field  can  dictate  this  direction  only 
when  the  shunt  field  is  broken.  Whether  the  motor 
starts  up  in  the  reverse  direction  or  not,  depends  upon 
whether,  with  the  weakened  field  and  the  current  flowing, 
there  is  torque  enough.  This  test  is  safe  to  try  only  at 
a  very  low  speed  and  at  a  voltage  insufficient  to  throw 
the  belt  on  the  loss  supplier,  even  when  the  motor  stands 
still.  The  rush  of  current  upon  breaking  the  shunt 
field,  where  the  two  fields  oppose  each  other,  is  greater 
than  that  at  starting,  the  voltage  on  the  loss  supplier 
being  the  same  in  both  cases,  because  it  is  a  fact, 
that  the  current  which  flows  at  the  time  of  breaking 
the  shunt  field  is  due  to  the  sum  of  the  impressed 
E.  M.  F.  and  what  was  the  C.  E.  M.  F.  of  the  motor, 
while  at  starting,  it  is  due  simply  to  the  impressed. 
If,  then,  upon  breaking  the  shunt  field  the  motor 
slows  down  and  stops,  the  series  field  connections 
must  be  reversed.  If  on  the  contrary  the  speed  rises, 
the  connections  are  correct.  Another  method  of  testing 
the  connections  is  to  start  up  with  the  series  windings 
short  circuited:  the  field,  and  hence  the  direction  of 


COMPOUNDING.  389 

rotation,  will  then  be  due  to  the  shunt  winding  alone;  if 
while  the  motor  is  turning  over  slowly  the  short  circuit 
be  removed,  the  series  winding  will  take  effect,  and  will, 
if  the  connections  are  proper,  reduce  the  speed,  because, 
if  connected  to  assist  the  shunt  winding,  its  introduction 
will  strengthen  the  total  field,  raise  the  C.  E.  M.  F., 
lower  the  current  flowing,  and  with  it  the  speed.  If, 
however,  the  connections  are  such  as  to  oppose  the  two 
windings,  the  effect  of  introducing  the  series  winding  is 
to  weaken  the  field  and  raise  the  speed.  In  the  hands 
of  a  careful  tester  the  above  methods  are  satisfactory 
and  are  quickly  carried  out.  Ordinarily  it  is  safer  to 
resort  to  the  galvanometer  test  already  given,  but  if  there 
are  no  facilities  for  doing  so,  the  speed  test  is  recom- 
mended but  with  one  injunction — use  as  slow  a  speed  as 
possible;  otherwise  a  belt  will  fly  off  whether  connections 
are  right  or  wrong. 

A  modification  of  the  above  tests  of  connections  consists 
in  varying  the  strength  of  the  shunt  field  by  means  of  its 
rheostat  and  noting  its  effect  upon  the  speed.  If  the  wind- 
ings are  concurrent  the  speed  will  rise  as  the  shunt  field 
is  weakened,  and  will  fall  when  it  is  strengthened.  This 
test  is  given  because  in  experienced  hands  it  has  served 
its  purpose,  but  it  is  interesting  to  consider  the  condi- 
tions under  which  its  results  would  be  very  misleading: 
i.  The  series  winding  partly  or  wholly  short  circuited: 
in  this  case  the  shunt  field  would  be  practically  the  total 
field,  and  the  right  or  wrong  series  field  connection  could 
give  no  indication;  2.  If  the  shunt  field  were  so  much 
stronger  than  the  series  field  that  the  difference  between 
the  two  were  greater  than  the  series  field  alone,  the  effect 
of  varying  the  box  between  certain  limits  would  be  the 


390  TESTING    OF    DYNAMOS    AND    MOTORS. 

same  as  if  no  series  field  existed.  This  test  is  not  recom- 
mended, because  its  indications  can  be  properly  interpreted 
only  when  the  operator  is  thoroughly  familiar  with  the  ma- 
chine under  test,  for  any  difference  in  degree  of  over-com- 
pounding involved  will  modify  the  behavior  very  much. 

If  the  machine  used  to  excite  the  motor  fields  is  used 
at  the  same  time  to  excite  other  fields  than  the  ones  in 
the  test,  the  several  circuits  should  be  made  independent 
by  placing  a  switch  in  each,  or  by  connecting  below  the 
exciter  switch  all  those  not  to  be  disturbed,  and  above 
this  switch,  those  circuits  which  it  may  be  necessary  to 
to  break  at  intervals.  Opening  the  switch  will  then 
break  only  the  latter. 

When  a  compound-wound  motor  runs  free,  its  series 
winding  contributes  but  little  to  the  magnetization,  since 
there  is  the  minimum  current  in  the  series  coils,  but  the 
influence  increases  with  the  load,  till  a  point  may  be 
reached  where  the  series  winding  has  as  great  an  effect 
as  the  shunt,  and  at  this  point,  if  the  two  windings  are 
in  opposition,  they  will  neutralize  each  other,  with  a 
resulting  short  circuit.  Neutralization  will  also  take 
place,  when,  for  the  purpose  of  increasing  the  speed,  the 
shunt  field  is  weakened  by  means  of  its  rheostat;  such  a 
short  circuit  can  only  take  place  where  the  two  windings 
are  in  opposition — hence  the  importance  of  getting  them 
right  before  hand.  However,  a  comparative  short  cir- 
cuit follows  a  sudden  great  weakening  of  the  field  in  all 
cases;  as,  for  instance,  the  breaking  of  the  shunt  field  in 
the  above  test  of  connections  reduces  the  motor  to  a 
series  motor  with  a  much  lighter  field  than  when  the  shunt 
field  acts  also.  The  result  is  that  the  reduced  C.  E.  M.  F. 
allows  an  abnormal  current  flow,  and  if  the  armature  is 


COMPOUNDING.  391 

heavy,  so  that  its  speed  responds  too  slowly  to  the  new 
conditions,  a  belt  flies  off  of  some  machine. 

Immediately  upon  closing  the  switch,  at  starting,  the 
brushes  of  the  supplier,  if  of  copper,  must  be  brought 
forward,  to  prevent  sparking.  The  supplier's  field  must 
be  strengthened  or  trouble  will  follow,  because,  when 
the  switch  is  first  closed,  the  compound-wound  loss  sup- 
plier  is  virtually  short  circuited,  having  in  series  with  it 
the  armature  of  its  companion  (if  there  are  two  suppliers 
in  series)  and  the  stationary  armature  of  the  motor;  the 
initial  flow  of  current  is  therefore  considerable,  and  the 
series  field  heavy.  The  result  of  this  is  that  the  E.  M.  F. 
of  the  supplier  rises  rapidly,  and  as  this  is  the  E.  M.  F. 
impressed  at  the  motor  terminals,  the  motor  speed  rises 
also,  and  with  it  its  C.  E.  M.  F.  Now,  since  the  motor  is 
separately  excited,  its  C.  E.  M.  F.  is  independent  of  the  im- 
pressed E.  M.  F. ,  except  in  so  far  as  the  speed  is  influenced 
by  it.  The  heavy  motor  armature  having  once  gained 
headway,  will,  by  virtue  of  its  large  inertia,  tend  to  hold 
its  speed,  and  will  not  quickly  respond  to  changes  in  the 
impressed  E.  M.  F.  Initially,  the  heavy  series  field  on 
the  compound-wound  loss  supplier  enables  it  to  generate 
a  high  E.  M.  F.,  but  as  the  C.  E.  M.  F.  rises,  the  current 
in  the  motor  circuit,  and  hence  in  the  series  field  of  the 
supplier,  falls  off  rapidly,  and  may  fall  so  low  that  the 
impressed  E.  M.  F.  no  longer  exceeds  the  C.  E.  M.  F. 
In  other  words,  the  motor  armature  becomes  a  generator 
running  by  its  own  momentum,  and  sends  a  current  back 
through  the  loss  supplier,  running  it  as  a  motor.  The 
preventative  of  such  behavior  is  to  strengthen  the  shunt 
field  of  the  loss  supplier  so  promptly  that  reversal  can- 
not take  place. 


392  TESTING    OF    DYNAMOS    AND    MOTORS. 

The  action  of  the  system  under  the  above  con- 
dition may,  if  the  motor  armature  be  very  heavy, 
so  that  it  cannot  quickly  come  to  rest,  become  quite 
complicated,  and  it  will  be  instructive  to  follow  the 
cycle  through.  As  soon  as  the  C.  E.  M.  F.  exceeds  the 
impressed,  thus  making  the  motor  a  generator  feeding 
the  supplier,  the  current  in  the  series  coils  of  the  supplier 
is  reversed,  and  so  is  the  polarity  of  its  fields.  When  the 
shunt  field  picks  up  anew  (if  it  does),  it  does  so  in  accord- 
ance with  the  reversed  polarity,  and  gives  us  the  condition 
of  both  machines  running  in  series  as  dynamos,  and  with 
no  resistance  in  circuit  save  the  two  armatures  and  two 
series  fields.  Both  machines  are  doing  work:  the  loss 
supplier,  being  connected  to  the  engine,  can  do  work  as 
long  as  the  belt  stays  on;  but  the  motor  armature,  owing 
its  energy  solely  to  momentum,  stops  very  soon.  This 
brings  us  to  a  new  stage.  The  loss  supplier  has  a  strong 
series  field  of  reverse  polarity  to  what  it  had  at  the  start, 
and  hence  sends  a  reverse  current  through  the  motor 
armature;  the  motor  fields,  being  separately  excited,  are 
the  same  as  at  starting,  therefore  as  soon  as  the  motor 
armature  comes  to  rest,  since  its  fields  are  of  the  same 
polarity  as  before  but  that  of  its  armature  is  reversed,  it 
begins  to  rotate  again,  but  in  the  opposite  direction.  If 
belts,  fuses,  and  circuit-breakers  hold  intact,  this  cycle 
of  operations  will  repeat  itself,  the  original  condition 
of  affairs  recurring  every  second  reversal.  Unless  the 
machines  are  allowed  to  reverse  themselves  a  second 
time,  the  shunt  field  of  the  motor  must  be  reversed  before 
the  system  will  rotate  in  the  proper  direction.  It  must 
also  be  remembered  that  since  the  two  suppliers  were  in 
series  at  the  start,  now  that  one  of  them  has  been 


COMPOUNDING.  393 

reversed  the  other  must  also  be,  so  they  will  again  be 
in  series. 

Promptness  and  care  are  the  preventatives  of  the 
above  complications,  the  compound-wound  supplier's 
brushes  being  brought  forward,  and  its  shunt  field 
resistance  reduced  so  as  to  counterbalance  the  weaken- 
ing of  the  series  field.  As  the  motor  speed  rises  and  its 
armature  current  decreases,  the  supplier  brushes  are 
worked  back  to  their  non-sparking  point. 

It  is  practicable  and,  in  some  tests  requiring  nice  volt- 
age regulation,  desirable  to  have  the  voltages  of  the  two 
suppliers  in  opposition.  The  minimum  line  voltage  is 
then  gotten  when  the  two  machines  are  of  equal  voltage. 
In  the  present  case  we  weaken  the  field  of  the  low  voltage 
machine  when  it  is  desired  to  raise  the  impressed  E.  M.  F. 
The  practice  of  opposing  E.  M.  Fs.  as  a  means  of  reg- 
ulation is  fast  gaining  ground  now  occupied  by  ordinary 
ohmic  resistances,  which  waste  so  much  energy  in  heat. 
The  flexibility  of  the  method  is  readily  seen  when  we  say 
that  with  two  500  volt  generators,  their  rheostats,  and  a 
reversing  switch  in  one  of  the  circuits,  any  voltage  is 
obtainable  from  o  to  1000. 

If  after  full  speed  is  attained  it  is  found  to  be  too  high, 
it  can  be  reduced  by  lowering  C's  E.  M.  F.  In  doing 
this  great  care  must  be  exercised  that  it  is  not  done  too 
suddenly,  and  a  reversal  precipitated.  Reversals  at  high 
speeds  are  generally  accompanied  by  flying  belts  and 
grand  confusion. 

With  B  running  up  to  speed,  but  without  load,  it  is 
time  to  introduce  the  low  volt  supplier  into  circuit. 
Since  its  armature  is  always  in  circuit,  to  make  the 
machine  active  it  is  only  necessary  to  complete  its 


394  TESTING    OF    DYNAMOS    AND    MOTORS. 

separately  excited  field  circuit,  and  gradually  increase  its 
field  strength,  at  the  same  time  decreasing  that  of  the 
compound-wound  machine  by  about  the  same  amount. 
The  voltage  of  the  low  volt  machine  is  brought  up 
to  the  maximum  to  be  used,  so  it  will  require  no 
further  regulation,  and  that  of  the  compound-wound 
machine  is  reduced  till  the  sum  of  the  two  is  that 
required.  The  compound-wound  machine  is  then  used 
for  all  further  regulation.  The  compound-wound  machine 
is  provided  with  field  rheostats  enough  to  cause  it  to 
drop  its  field,  when  all  resistance  is  in,  and  the  low  volt 
machine's  field  must  have  resistance  enough  to  reduce 
its  E.  M.  F.  to  30  or  40  volts,  which  will  be  necessary 
both  when  putting  the  machine  into  action  and  withdraw- 
ing it.  In  the  first  place  to  prevent  the  introduction  of  its 
voltage  from  causing  too  great  an  increase  in  the  load; 
in  the  second  'place  to  avoid  the  withdrawal  of  its 
E.  M.  F.  from  causing  a  reversal. 

We  next  prepare  to  load  A  on  B.  If  A  and  ^are  belt- 
connected,  A  turns  over  when  B  does;  but  if  clutched 
together  it  is  customary  to  put  in  A's  clutch  after  B  is 
up  to  speed,  an  operation  which  requires  some  care, 
because  A's  heavy  armature  has  considerable  inertia  and 
acts  as  a  brake,  and  may  temporarily  overload  the  com- 
pound-wound supplier  sufficiently  to  throw  Its  belt.  The 
low  volt  machine  gives  no  trouble,  because,  although  it 
runs  at  full  voltage,  it  is  far  from  its  normal  current 
output  and  is  therefore  nowhere  near  its  full  load. 

The  next  step  is  to  get  the  voltage  on  the  motor  at  ap- 
proximately 500  volts,  at  the  same  time  keeping  the  speed 
right  by  means  of  the  resistance  boxes  in  its  field.  Sup- 
posing that  the  voltage  is  correct,  and  that  the  speed  is 


COMPOUNDING.  395 

low;  bringing  up  the  speed  by  weakening  B's  field  will 
cause  the  line  current,  and  hence  "lost volts,"  to  increase, 
and  hence  also  the  voltage  impressed  on  B,  unless  C  is 
a  single  machine  perfectly  compounded  for  the  occasion. 
Voltage  and  speed  must  therefore  be  adjusted  simul- 
taneously. It  would  seem  at  first  sight  that  a  perfectly 
compounded  dynamo  might  be  useful  as  a  loss  supplier; 
and  so  it  would  for  either  no  load  or  full  load,  but  not  for 
both  and  intermediate  loads,  since  at  no  load  the  voltage 
must  be  500,  at  full  load  555,  and  at  intermediate  loads  be- 
tween these  limits.  The  only  recourse,  then,  is  to  hand 
regulation.  The  voltage  on  B  and  the  speed  of  A  are  ad- 
justed after  A  is  made  to  generate  its  own  field,  because 
the  making  of  A's  field  puts  more  work  on  B  and  brings 
down  its  speed.  To  insure  that  A's  polarity  shall  be 
that  desired,  its  field  is  charged  from  C,  by  removing  a 
brush  holder  cable  and  closing  K.  By  charging^  from 
C,  we  can  be  sure  that  their  polarities  shall  be  opposed, 
and  for  this  reason:  when  the  brush-holder  cable  is 
removed  from  A,  in  order  to  cut  its  armature  out  of  cir- 
cuit; and  K  is  closed,  A  and  B  may  be  regarded  as  two 
motors  connected  in  multiple  on  C.  The  two  shunt  fields 
are  charged  exactly  as  they  would  be  were  the  two 
machines  actually  running  as  motors.  Now  the  C.  E. 
M.  F.  of  a  motor  opposes  its  impressed  E.  M.  F.,  and 
regarding  A  and  B  as  motors,  the  impressed  E.  M.  F. 
being  common  to  both,  is  the  same  on  both,  and  since 
both  C.  E.  M.  Fs.  are  opposed  to  the  impressed  E.  M.  F. 
they  are  opposed  to  each  other,  when  we  consider  the 
local  circuit  of  the  two  machines,  as  will  be  seen  in  Figs. 
128  and  129.  Here  Fig.  128  shows  the  condition  while 
charging,  and  Fig.  129  the  same  after  the  impressed 


396  TESTING    OF    DYNAMOS    AND    MOTORS. 

E.  M.  F.  has   been   removed,   leaving  the  two  machines 
opposed  in  polarity. 

As  soon  as  A's  field  is  excited,  and  its  voltage  adjusted 
to  500,  its  brushes  are  brought  as  far  forward  as  spark- 
ing will  permit:  it  will  then  be  unnecessary  to  shift  them 
when  the  load  is  put  on.  With  voltage  at  500  and  speed 
correct  and  constant,  we  are  ready  for  the  free  data, 
This  consists  of  voltage  and  speed  readings,  current  in 
shunt  field,  and  drop  across  series  coils,  all  with  no  load 
on  the  machine,  and  according  to  blanks  provided 

'C.E.M.F. 


Imp.  EJd.F 

^N^ 

.C.E.M.F, 
FIG.    128. 

for  the  test.  The  "free"  data  taken,  A's  E.  M.  F. 
is  500  volts,  and  that  on  B  a  little  less.  The  volt  lines 
are  now  placed  across  K,  which  is  the  only  open  cir- 
cuit between  the  two  machines,  and  the  voltmeter  should 
register  zero,  showing  that  the  two  sides  of  the  switch 
are  at  the  same  potential.  In  holding  the  voltlines 
the  operator  must  have  care  lest  there  be  a  potential 
difference  of  1,000  volts,  which  is  the  case  if  A  and  B 
happen  to  be  in  series;  should  this  be  the  case,  A's  field 
must  be  recharged  from  C.  Inability  to  get  full  voltage 
from  A  when  running  up  to  speed  indicates  one  or  more 
field  spools  to  be  wrongly  wound  or  connected, .  so  that 
one  spool's  magnetizing  effect  neutralizes  that  of  another. 
This  can  be  caused  either  by  getting  the  field  spools  in 
the  frame  end  for  end,  or  by  the  winder  bringing  the 
leads  around  one-half  turn  too  far  before  bringing  them 


COMPOUNDING.  397 

out:  either  of  which  mistakes  results  in  the  inside  field 
lead  coming  out  where  the  outside  lead  should  be.  A 
simple  test  is  to  bring  a  hand  compass  up  to  the  pole- 
pieces  and  observe  which  pole  of  the  needle  is  attracted: 
should  three  consecutive  poles  prove  to  be  alike,  the  mid- 
dle spool  must  be  reversed, either  electrically  or  mechanic- 
ally. In  lieu  of  a  compass  a  piece  of  soft  iron  will  serve 
as  well,  being  simply  held  before  the  poles,  and  passed 
freely  from  one  to  the  other.  Where  adjacent  poles  are 
unlike,  as  they  should  be,  the  piece  of  iron  will  follow  a 
natural  path  from  one  to  the  other,  presenting  opposite 
ends  to  adjacent  poles.  If  the  poles  are  alike,  the  iron 
will  tend  to  balance  midway  between  the  two  poles,  and 
present  the  same  end  to  both. 

We  have  now  arrived  at  perhaps  the  most  difficult,  and 
certainly  the  most  interesting,  part  of  the  test — that  of 
putting  on  the  load.  With  both  sides  of  A",  Fig.  123,  at  the 
same  potential,  or  with  a  slight  difference  in  favor  of  A's 
side,  it  can  be  closed.  At  this  time  there  should  be  a 
man  at  Cs  boxes;  one  to  take  speed  on  A;  one  at  £'s 
boxes;  one  at  B's  brushes  to  put  on  the  load;  and  last, 
but  by  no  means  least,  the  man  with  the  voltlines.  As 
soon  as  K  is  closed,  ^'s  field  is  weakened  a  little  to 
minimize  chances  of  reversal.  £'s  brushes  are  then 
brought  slowly  backward  till  about  one-quarter  load 
works  on.  As  the  load  goes  on,  Cs  voltage  is  raised  by 
means  of  its  rheostat,  in  order  to  keep  up  the  speed, 
cutting  out  field  resistance  slowly,  and  giving  each 
change  time  to  have  its  full  effect.  The  tachometer,  or 
instantaneous  speed  indicator,  should  be  checked  up 
with  a  timepiece  and  ordinary  indicator,  and  the  speed 
must  be  kept  exactly  right,  as  a  difference  of  4  or  5,. 


398  TESTING    OF    DYNAMOS    AND    MOTORS. 

revolutions  will  sometimes  cause  an  error  of  6  or  8  volts. 
The  tachometer  should  be  handled  carefully,  and  with- 
out leaning  on  it,  as  undue  pressure  causes  it  to  run  hot 
and  stick.  Speed  must  always  be  taken  on  the  dynamo 
and  not  on  the  motor,  for  although  they  are  belted  or 
clutched  together,  a  difference  in  pulleys  or  slipping  of 
belt  or  clutch  would  introduce  an  error. 

If  A  and  .#  are  heavily  over-compounded,  it  is  customary 
to  use  a  temporary  shunt  on  A's  series  field  to  prevent 
the  load  from  going  on  too  suddenly.  When  the  load 
reaches  quarter  value  this  shunt  is  slowly  worked  out,  and 
the  load  further  increases.  The  removal  of  the  shunt 
has  the  following  effect:  Primarily,  part  of  the  load  is 
put  on  by  reducing  j9's  C.  E.  M.  F.  This  is  accomplished 
by  rocking  the  motor  brushes  back  so  as  to  bring  the 
armature  poles  in  a  position  to  demagnetize  the  fields. 
The  effect  of  this  reaction  is  so  great  that  at  full  load 
when  the  field  is  weakest  the  field  current  is  often  greatest. 
On  a  dynamo  the  effect  of  giving  the  brushes  a  forward 
or  positive  lead  is  to  have  the  armature  reinforce  the 
field  and  raise  the  E.  M.  F.,  and  since  C.  E.  M.  F.  is 
the  dynamo  property  of  a  motor,  the  effect  of  rocking 
the  motor  brushes  forward  is  to  raise  its  C.  E.  M.  F.  and 
diminish  the  load.  The  full  load  can  now  be  worked  on 
A,  and  with  its  speed  and  load  adjusted  it  is  permitted  to 
run  5  hours,  or  till  it  heats  thoroughly.  One  man  can 
easily  regulate  the  load  and  watch  the  bearings. 

We  will  now  consider  the  factors  entering  into  the 
problem  of  compounding,  and  the  various  points  lending 
aid  to  success.  Compounding  consists  in  experimentally 
adjusting  a  permanent  German  silver  shunt  across  the 
series  field  of  the  dynamo,  such  that  when  running  at  full 


COMPOUNDING.  399 

load,  proper  speed,  and  fully  heated,  the  dynamo  shall 
give  a  specified  E.  M.  F.  At  the  beginning  of  the  test 
a  variable  shunt  board  is  used,  for  convenience,  and 
it  is  this  board  that  the  regulation  shunt  replaces  when 
the  load  has  been  removed  for  readjusting  the  rheostat 
for  500  volts  just  before  compounding.  So  far  as  the 
shunt  winding  is  concerned,  any  properly  designed 
machine  that  compounds  cold,  /.  <?.,  maintains  its  speci- 
fied voltage  from  no  load  to  full  load,  will  compound  hot, 
as  resistance  is  taken  from  the  rheostat  to  compensate 
for  the  rise  of  resistance  in  the  winding  due  to  heating. 
With  the  series  field,  however,  such  is  not  the  case.  If  a 
machine  be  compounded  cold  with  a  shunt  of  certain 
resistance,  it  will,  when  heated  and  its  rheostat  read- 
justed for  500  volts  on  open  circuit,  undercompound, 
and  a  new  shunt  will  be  required.  The  reason  for  this 
is  that  the  same  relation  between  the  series  field  and  the 
shunt  no  longer  exists.  In  the  first  place  the  shunt,  being 
exposed  to  the  air,  does  not  rise  very  much  in  tempera- 
ture; secondly,  German  silver  does  not  rise  in  resistance 
for  a  given  increase  in  temperature  at  the  same  rate  as 
copper  does,  the  rate  being  much  less.  The  consequence 
is  the  shunt  does  not  rise  very  much  in  resistance,  while 
the  series  field  does,  thereby  sending  a  greater  proportion 
of  the  armature  current  through  the  shunt  and  weakening 
the  field.  To  restore  them  to  their  former  relation  the 
shunt  resistance  is  raised,  by  cutting  out  one  or  more 
strips  of  German  silver.  The  only  factor  uncompensated 
directly  is  the  increased  reluctance  of  the  heated  iron; 
this  is  small. 

As  has  been  said  elsewhere,  the  present  test  has  been 
selected  to  include  as  many  troubles  as  possible,  but  it 


400  TESTING    OF    DYNAMOS    AND    MOTORS. 

must  not  be  supposed  that  all  of  them  occur  in  any  one 
test.  Few  of  them  should,  but  the  operator  must  be 
prepared  for  any. 

Should  the  load  go  on  with  a  rush  when  the  motor 
man  shifts  the  brushes,  it  indicates  any  of  the  following 
troubles:  (i)  Opposition  of  motor  fields;  (2)  absence 
of  shunt  on  dynamo  series  field;  (3)  excessive  over- 
compounding  of  the  dynamo;  (4)  shunt  on  the  motor 
series  field — probably  left  there  when  the  machines 
were  changed  over;  (5)  someone  dressing  a  belt  without 
giving  notice;  from  the  last  cause  alone  the  writers 
have  seen  the  ammeter  needle  go  steadily  but  rapidly 
from  quarter  load  to  overload.  The  effect  of  the  above 
cases  may  be  briefly  analyzed:  (i)  If  j9's  windings  are 
opposed,  any  weakening  of  the  shunt  field  results  in 
sending  current  through  the  series  winding  in  such  a 
direction  as  to  neutralize  and  further  weaken  the  field, 
and  as  the  current  increases  this  effect  multiplies,  and 
the  load  goes  on  too  fast  to  control.  To  test  polarity, 
the  usual  galvanometer  test  is  resorted  to.  (2)  The  ab- 
sence of  a  dynamo  shunt  permits  its  E.  M.  F.  to  rise  too 
rapidly  and  precipitates  the  load.  (3)  When  the  dynamo 
is  highly  over-compounded,  even  with  the  shunt  ordi- 
narily used,  the  series  field  effect  is  too  strong,  and  pre- 
cipitates the  load  for  the  same  reasons  as  given  in  (2), 
but  to  a  less  degree.  (4)  Weakening  the  motor  field  is 
the  same  in  effect  as  strengthening  that  of  the  dynamo. 
(5)  Tightening  the  belt  or  dressing  it  prevents  slipping 
and  raises  the  dynamo  speed  and  voltage,  thereby  in- 
creasing the  load. 

Should  the  load  fail  to  go  on,  it  may  be  due  to  any  of 
the  following  causes:  (i)  The  dynamo  series  field  may 


COMPOUNDING.  4OI 

be  too  powerfully  shunted;  (2)  the  dynamo  series  and 
shunt  fields  may  be  opposed;  (3)  the  ammeter  needle 
may  be  "  frozen";  (4)  a  belt  or  clutch  may  be  slipping,  (i) 
The  effect  is  to  rob  the  series  winding  of  current,  and 
thereby  deprive  it  of  ability  to  force  a  load  against  the 
growing  opposition  of  the  motor  series  winding.  Gradu- 
ally removing  the  shunt  will  show  if  this  is  the  cause. 
Virtually  the  same  effect  obtains  if  the  series  field  has 
a  loose  connection,  because  nearly  all  the  current  then 
passes  through  the  shunt.  In  this  case,  the  shunt  grows 
hot.  (2)  If  shunt  and  series  oppose,  neutralization  takes 
place  as  the  load  increases,  and  it  refuses  to  exceed  a 
certain  value.  The  series  connection  must  be  reversed. 
(3)  In  this  case  the  load  is  really  going  on,  though  the 
ammeter  does  not  record  the  fact;  tapping  the  meter  so 
as  to  free  the  needle  is  sufficient.  (4)  A  slipping  belt 
generally  announces  its  condition  by  squeaking,  but  not 
always.  As  an  illustration  of  belt  troubles  the  following 
very  singular  performance  may  be  interesting:  Two  500 
kilowatt  street  railway  generators  were  under  test,  and 
quarter  load  was  on,  after  which  all  efforts  to  increase  it 
were  futile.  Upon  weakening  the  motor  field  the  load 
decreased  instead  of  increasing.  The  action  suggested 
that  possibly  the  supposed  motor  was  really  a  generator, 
but  weakening  the  other's  field  disproved  this.  Finally  the 
trouble  was  removed  by  tightening  the  belt.  The  behavior 
is  explained  as  follows :  upon  weakening  the  motor  field  its 
speed  became  higher,  but  contact  between  pulley  and  belt 
became  poorer  at  the  higher  speed,  so  that  the  dynamo 
speed  became  actually  less.  Upon  strengthening  the 
motor  field  the  speed  fell  off,  but  its  belt  grip  improved, 
so  that  the  dynamo  speed  increased  and  with  it  the  load. 


4O2  TESTING    OF    DYNAMOS    AND    MOTORS. 

There  was  naturally  a  limit  to  this  unusual  method 
of  putting  on-  a  load,  for  with  good  belt  contact, 
further  strengthening  of  the  motor  field  brought  clown 
the  speed  on  both  machines,  and  also  the  impressed 
and  counter  E.  M.  Fs.,  but  the  former  faster  than  the 
latter.  The  reason  for  this  is:  (i)  The  dynamo,  being 
self-exciting,  decreasing  its  speed  not  only  decreases  its 
E.  M.  F.  directly,  but  does  so  further  by  weakening  the 
field  which  depends  upon  that  E.  M.  F.,  an  effect  absent 
on  the  separately  excited  motor.  (2)  Since  the  dynamo's 
E.  M.  F.  exceeds  the  motor's  counter  by  an  amount  equal 
to  the  cable  "  drop,"  a  percentage  reduction  in  the  speed 
causes  a  greater  absolute  effect  on  the  dynamo,  so  that 
its  excess  of  voltage  gradually  disappears,  and  with  it 
its  load. 

Should  the  difficulty  in  putting  on  the  load  not  be 
found  among  the  above  more  common  ones,  it  may  be 
due  to  the  rocker  arm  on  one  machine  being  carelessly 
left  90°  out  of  position,  which  results  in  reversing  the 
series  field  of  the  motor,  and  in  depriving  the  generator 
of  its  ability  to  generate  unless  its  shunt  field  has  been 
reversed  to  suit  conditions,  in  which  case  the  series  field 
must  be  also  reversed.  No  sane  man  is  apt  to  leave 
brushes  out  of  position  if  allotted  reasonable  time  for  his 
work,  because  the  handle  of  the  rocker  arm  is  a  glaring 
indicator  of  its  position,  but  when  eight  or  ten  men 
work  on  the  same  test  trying  to  do  half  an  hour's  work 
in  five  minutes,  almost  anything  is  liable  to  occur. 

There  are  times  when  the  ammeter  needle  surges 
back  and  forth  from  no  load  to  full  load.  To  locate  the 
cause  of  such  action  it  is  necessary  to  note  simultane- 
ously the  variation  of  load  and  speed.  If  the  two  rise 


COMPOUNDING.  403 

and  fall  together,  the  trouble  is  with  the  loss  supplier, 
or  some  belt,  clutch,  engine,  or  countershaft  connected 
with  it.  If  the  speed  falls  when  the  load  rises,  it  indi- 
cates trouble  in  the  exciter  circuit,  for  we  have  seen 
that  weakening  the  motor  field  increases  the  load  and 
decreases  the  speed,  and  vice  versa;  whije  raising  the 
voltage  on  the  suppliers  increases  the  load  and  speed  at 
the  same  time.  The  proof  of  a  slipping  belt  is  a  warm 
pulley. 

When  there  is  persistent  trouble  with  the  speed  and 
careful  inspection  fails  to  class  it  in  any  of  the  above 
cases,  suspicion  points  strongly  to  the  motor,  and  is  con- 
firmed by  the  fact  that  the  speed  is  equally  unsteady  at 
no  load  and  full  load.  If  the  trouble  is  with  the  fields, 
as  is  apt  to  be  the  case,  the  speed  will  rise  as  the  load 
falls.  A  rough  test  is  made  by  means  of  a  piece  of  soft 
iron  and  a  compass;  the  iron  showing  any  marked 
discrepancy  in  pole  strength  and  the  compass  any 
irregular  polarity.  If  the  trouble  is  with  the  armature, 
the  indications  are  apt  to  be  more  violent,  and  give 
rise  to  local  heating.  A  good  way  to  determine  exactly 
the  seat  and  nature  of  its  trouble  is  to  reverse  the 
machines  and  run  the  defective  motor  as  a  dynamo:  if 
the  trouble  is,  as  suspected,  with  the  motor,  the  incon- 
stancy of  speed  will  no  longer  exist.  Next  see  if  the 
defective  machine  will  excite  its  own  field,  and  if  not, 
charge  from  the  motor  terminals:  the  field  ammeter  will 
show  by  the  magnitude  of  the  deflection  whether  the 
field  resistance  is  normal  or  not.  Lastly  take  the  drop 
of  potential  across  each  spool  and  locate  the  faulty  one. 

With  all  difficulties  surmounted  and  the  full  load  on, 
the  machines  are  allowed  to  run  until  thoroughly  heated, 


404  TESTING    OF    DYNAMOS    AND    MOTORS. 

which  requires  from  three  to  eight  hours.  The  load  is 
then  removed,  and  the  dynamo  field  rheostat  readjusted 
for  500  volts  on  open  circuit  and  the  shunt  board  replaced 
by  eight  or  ten  strips  of  German  silver  tape.  The 
machine  is  then  compounded  according  to  the  directions 
on  the  test  sheet.  With  full  load  on  and  correct  speed, 
the  voltage  must  be  that  of  the  specified  over-compound, 
which  in  this  case  we  assume  to  be  555  volts.  If  on  the 
first  arrangement  of  the  German  silver  shunt  the  full  load 
voltage  is  too  high,  the  shunt's  length  can  be  diminished 
by  drawing  it  through  the  clamps,  thereby  lowering  its 
resistance,  if  the  voltage  is  too  low  the  cross-section  can 
be  lessened  by  taking  out  a  strip.  Care  must  be  take"n 
that  throughout  the  process  the  speed  and  current  are 
kept  right.  The  "up"  and  "down"  readings  of  voltage 
are  now  taken,  and  the  former  should  be  uniformly  lower 
than  the  latter.  If  this  is  not  the  case,  it  shows  that 
either  there  is  an  error  in  one  set  of  readings,  or  that  the 
machine,  not  having  reached  its  maximum  temperature,  is 
•still  rising.  Having  passed  this  final  test,  the  compound 
is  completed,  and  the  change  over  is  in  order.  The 
German  silver  shunt  is  removed,  carefully  labeled,  and 
sent  to  be  made  up  in  compact  form.  The  series  field 
resistance  is  measured,  hot  and  cold,  on  both  machines, 
and  from  these  data  is  figured  the  actual  temperature  rise. 
If  there  is  an  ammeter  in  the  supplier  circuit  we  know 
the  total  motor  current,  as  well  as  that  of  the  dynamo, 
and  by  taking  the  drop  on  the  respective  series  fields  we 
can  at  any  time  tell  their  resistances.  In  making  these 
measurements  care  must  be  taken  that  the  fields  are  not 
shunted,  for  the  multiple  resistance  of  coils  and  shunt  is 
much  less  than  that  of  either  alone.  Should  the  drop 


COMPOUNDING.  405 

be  abnormally  high,  that  across  each  spool  must  be 
taken,  and  in  such  a  way  as  to  include  no  joints;  the 
drop  across  each  joint  can  then  be  taken,  and  any  loose 
or  lacquered  joint  located. 

The  above  data  having  all  been  taken,  the  machines 
are  shut  down  and  changed  over,  the  motor  becoming  a 
dynamo,  and  vice  versa.  The  change  consists:  (i)  In 
removing  the  supplier  cable  next  to  A",  and  placing  it  on 
the  other  side  of  K'  (Fig.  123);  (2)  in  exchanging  the  box 
lines  of  the  two  machines,  and  making  the  motor,  to  be, 
separately  excited  and  the  dynamo  self-excited.  In  most 
cases  the  series  fields  must  be  reversed,  but  if  A  and  B 
are  belted  together,  and  one  is  provided  with  right-hand 
the  other  with  left-hand  brush  holders,  or  if  the  ma- 
chines are  clutched  together  and  both  are  fitted  with 
similar  brush  holders,  then  their  direction  of  rotation 
must  be  reversed  to  conform  to  the  rule  requiring  car- 
bon brush  dynamos  to  run  against  the  brushes,  and  the 
series  field  connections  need  not  be  disturbed  except  to 
transfer  the  fuse  from  the  old  motor  to  the  new  one. 
The  shunt  board  must  also  be  transferred  to  the  dynamo 
to  be.  Due  regard  must  be  had  for  the  fact  that  machines 
belted  together  generally  turn  in  the  same  direction, 
viewed  from  the  commutator  end,  but  when  clutched 
together  they  stand  pulley  to  pulley,  and  therefore  turn 
oppositely.  The  change  in  connections  completed,  the 
machines  are  once  more  started  up,  and  allowed  to  run  for 
a  few  moments  before  putting  on  the  load.  In  throwing 
in  a  clutch,  after  it  has  been  snapped,  the  pressure  ring 
must  be  relieved,  otherwise  it  will  run  hot.  Any  slipping 
can  be  detected  by  taking  speed  on  both  machines.  The 
load  is  put  on  as  already  described,  and  the  system 


406  TESTING    OF    DYNAMOS    AND    MOTORS. 

allowed  to  run  for  three  or  more  hours  longer;  when  the 
second  machine  is  compounded  the  set  is  run  long 
enough  to  complete  the  ten  or  twelve  hour  test  as  the 
case  may  be. 

If  after  a  load  has  been  running  smoothly  for  some 
hours  it  begins  to  work  off  slowly,  the  cause  is  either  a 
slipping  belt  or  clutch,  or  more  likely  a  movement  of  one 
or  both  rocker  arms.  This  can  be  detected  at  a  glance 
by  making  a  chalk  mark  on  pillow  block  and  rocker  arm  at. 
the  beginning  of  the  test.  There  is  a  tendency  for  the 
commutator  to  drag  the  brushes  and  rocker  arm  around 
with  the  armature,  and  no  dynamo  should  be  shipped  in  a 
condition  to  admit  of  this.  Testers  pay  special  attention 
to  this  detail,  for,  when  running  in  multiple  with  others,  a 
machine  with  such  a  weakness  would  be  sure  to  cause 
trouble.  The  movement  of  the  rocker  arm  on  either 
dynamo  or  motor  has  the  effect  of  working  off  the  load. 
On  the  dynamo  it  decreases  the  lead,  and  hence  the 
E.  M.  F.  On  the  motor  it  increases  the  negative  lead, 
and  hence  increases  the  C.  E.  M.  F.,  both  of  which 
actions  tend  to  remove  the  load.  The  first  symptom  of 
a  shifting  rocker  arm  is  a  decrease  in  the  ammeter  read- 
ing, followed  by  sparking  of  the  motor  brushes,  and 
unless  noticed  in  time,  the  load  works  down  to  zero, 
at  which  point  the  supplier  runs  both  machines  as 
motors,  and  the  dynamo  announces  the  fact  by  a  char- 
acteristic howl. 

Circumstances  sometimes  demand  the  instant  removal 
of  the  load,  by  pulling  switch  K:  in  such  a  case  the 
motor  brushes  should  be  brought  quickly  forward  to 
relieve  sparking,  the  motor  field  strengthened,  and  the 
supplier  E.  M.  F.  reduced,  otherwise  the  speed  will  rise 


COMPOUNDING.  407 

to  the  limit  set  by  the  motor's  separate  excitation.  The 
reason  is  as  follows:  Since  bringing  the  brushes  back  to 
put  the  load  on  has  lowered  the  C.  E.  M.  F.,  the  first  ten- 
dency  is  for  the  speed  to  rise,  but  this  is  met  by  the 
increased  load  which  the  over-compounded  dynamo 
immediately  puts  on  the  motor,  with  the  result  that  the 
speed  must  be  maintained  by  raising  the  supplier's 
E.  M.  F.  The  conditions  at  the  time  of  removing  the 
load  then  are  these:  (i)  The  supplier  voltage  is  much, 
above  the  value  necessary  to  keep  the  system  at  the 
given  speed,  had  the  dynamo  no  load;  (2)  the  powerfut 
series  field  of  the  motor  under  full  load  has  an  effect 
almost  negligible  as  soon  as  the  load  is  thrown  off.  The 
motor  brushes  are  in  a  position  corresponding  to  high, 
speed  at  no  load,  and  may  or  may  not  be  counter-bal- 
anced by  the  excess  of  shunt  field  current  necessary  to- 
reduce  sparking.  The  effect,  then,  of  removing  the  load 
is  to  remove  the  .restraining  influence,  hence  the  advis- 
ability of  the  above  precautions. 

A  simple  way  to  prevent  all  danger  is  to  pull  K  and  K' 
at  the  same  time:  this  has  the  disadvantage  that  it  stops 
the  system.  Pulling  K"  without  pulling  K  stops  the 
set,  but  not  without  sparking  badly,  for  it  leaves  A  and 
B  to  run  back  on  each  other  in  virtue  of  their  inertia, 
and  since  A  is  self-exciting,  it  will  lose  its  field  when  the 
speed  gets  below  the  critical  value,  leaving  A  to  short 
circuit  B,  now  running  as  a  separately  excited  generator. 

Pulling  the  main  switch  under  full  load  in  a  motor- 
generator  test  is  not  attended  by  serious  arcing — even 
with  a  current  of  2,000  amperes,  and  a  switch  with  a 
3  inch  break  is  safe.  Such  is  not  the  case  on  a  similar 
circuit  containing  only  ohmic  resistance,  for  here  a  switch 


408  TESTING    OF    DYNAMOS    AND    MOTORS. 

of  three  times  the  break  would  not  prevent  arcing.  The 
conditions  in  the  two  cases  are  different:  in  the  latter 
the  full  voltage  of  the  system  is  effective  in  supporting 
the  arc,  while  in  the  circuit  containing  the  motor's 
C.  E.  M.  F.,  only  the  excess  of  the  impressed  over  the 
counter  E.  M.  F.  is  so  available;  and  this  is  merely  that 
necessary  to  drive  the  current  through  the  low  ohmic 
resistance  of  the  machines  and  connecting  cables.  It  is 
good  practice  to  adhere  to  a  uniform  rule  in  removing  a 
load:  the  supplier's  E.  M.  F.  must  be  reduced  step  by  step 
with  that  of  the  dynamo,  and  the  motor  speed  kept  con- 
stant. This  is  continued  until  the  load  is  off,  and  when 
K  is  opened  everything  is  nearly  adjusted  to  put  back 
the  load  if  it  is  desired  to  do  so. 

The  test  over,  the  thermometer  temperature  of  arma- 
tures and  fields  is  taken,  and  the  hot  insulation  meas- 
ured. The  thermometer  is  placed  on  the  part  to  be 
taken,  and  is  well  packed  around  with  cotton  waste.  The 
armature  requires  half  an  hour  or  more  after  stopping  in 
which  to  reach  its  maximum  temperature,  because  while 
running  the  fanning  of  the  air  keeps  the  surface  layer  of 
wire  cooler  than  those  inside.  The  most  reliable  infor- 
mation in  regard  to  temperature  rise  is  gotten  from  the 
rise  in  resistance. 

Insulation  measurement  is  taken  from  series  to  shunt, 
series  to  frame,  shunt  to  frame,  and  commutator  to  shaft: 
we  specify  shaft  to  eliminate  any  possible  error  due  to 
the  film  of  oil  which  lies  between  the  shaft  and  frame. 
In  testing  the  fields  all  exterior  wires  are  disconnected, 
and  all  wires  remaining  are  cleared  from  the  frame.  One 
test  line  is  then  held  on  one  field  terminal,  the  other  test 
line  on  the  frame.  If  any  of  the  field  tests  are  low,  the 


COMPOUNDING'.  409 

spools  are  separated,  tested,  and  the  faulty  one  marked. 
A  low  spool  or  armature  is  returned  to  the  oven  and 
baked  till  it  passes  the  test. 

The  galvanometer  used  in  testing  insulation  is  a 
Thomson  reflector,  with  the  scale  at  such  a  distance  as 
to  introduce  no  appreciable  error  in  assuming  that  the 
deflection  produced  is  directly  proportional  to  the 
E.  M.  F.  applied  to  the  galvanometer.  The  galvanom- 
eter in  question  is  calibrated  with  a  Daniell  cell  of  i.i 
volt,  and  the  resistance  in  circuit  is  such  that  with  no 
shunt  1. 1  volt  produces  a  deflection  of  1 10  scale  divisions. 
Each  division  therefore  corresponds  to  .01  volt.  Using 
the  1/99  shunt,  one  division  corresponds  to  i  volt. 

By  combining  shunts  and  proportion  lines,  the  range  of 
voltage  possible  to  be  read  is  from  .01  to  500,000  volts. 
This  range  has,  however,  its  limit  in  the  carrying  ca- 
pacity of  the  proportion  boxes.  All  that  is  seen  and 
handled  of  the  proportion  box  is  two  pairs  of  small  flexi- 
ble cables  leading  from  it;  one  pair,  including  the  total 
resistance  of  the  box,  goes  to  the  terminals  of  the 
machines  under  test.  The  other  pair  includes  the  de- 
sired fraction  of  the  total  resistance  and  goes  to  the 
galvanometer.  Should  the  lines  be  confused,  resulting 
in  putting  the  fractional  resistance  across  the  source 
of  E.  M.  F.,  a  fuse  may  blow  unpleasantly  near  the 
operator's  eyes,  or  the  galvanometer  may  be  injured. 

In  testing  insulation,  the  galvanometer,  with  its  resist- 
ance boxes,  charging  lines,  safety  lamps,  and  test  lines 
are  all  in  series,  as  shown  in  Fig.  130.  The  1/99  or  1/999 
shunt  is  used  on  the  galvanometer  according  as  the 
charging  lines  are  from  the  125  or  500  volt  circuit.  The 
test  lines  are  first  held  together,  and  the  galvanometer 


4io 


TESTING    OF    DYNAMOS    AND    MOTORS. 


deflection  noted.  This  deflection  is  due  to  the  current 
which  A  sends  through  R,  G,  in  series,  and  is  called 
the  constant.  The  lines  are  now  held  apart,  and  no 

permanent  deflec- 
tion should  obtain 
if  the  insulation  is 
everywhere  intact. 
The  lines  are  now 
held  across  the  in- 
sulation to  be  meas- 
ured, care  being 
FIG.  130.  taken  that  the  fin- 

gers do  not   touch 

the  wires,  for  the  body  has  an  average  resistance 
of  but  5,000  ohms,  and  would  be  a  comparative  short 
circuit  across  the  insulation,  which  measures  perhaps 
megohms.  If  now  the  "constant"  is,  say,  135  volts, 
corresponding  to  a  deflection  of  135  divisions,  and 
the  insulation  reading  is  but  i  division,  the  circuit 
resistance  is  135  times  as  great  as  it  is  when  the  "test  " 
is  cut  out,  and  the  insulation  resistance  is  approximately 
135  times  that  of  the  galvanometer  boxes,  etc.  In  this 
way  the  insulation  resistance  is  gotten,  and  none 
below  1,000,000  ohms  (i  megohm)  is  accepted.  The 
following  table  gives  the  insulation  value  of  one  division, 
using  either  the  125  or  500  volt  charging  lines.  All 
instruments  used  in  these  tests  are  calibrated  at  regular 
intervals  by  comparison  with  reliable  standards. 

SHUNT   USED.     DEFLECTION.     MEGS.  @  125  VOLTS.     MEGS.  @  5OO  VOLTS. 

o  i  J35°  54°° 


i/99  i  !3-5  54 

i/999  I  i-35  5-4 


COMPOUNDING.  411 

TEST  XL — Compounding;  all  Machines  in  Series. — Test 
X  required  the  loss  supplying  generator  to  be  of  the 
same  E.  M.  F.  as  the  machines  under  test,  though 
its  current  capacity  might  be  considerably  less,  and  fur- 
ther required  that  it  run  in  multiple  with  the  dynamo. 
In  the  present  test  the  current  capacity  of  the  supplier 
must  equal  or  exceed  that  of  the  machines  under  test; 
and  all  machines  are  in  series,  as  far  as  having  the  cur- 
rent pass  out  of  one  into  the  other  fulfills  the  definition 
of  the  term.  Fig.  131  shows 

the   connections   for    test.          K  A/H    L    h 

A  is  a  compound-wound 
generator;  B,  a  compound- 
wound  machine  similar  to 
A  (series  fields  are  omitted  E 

in  figure),  to  be  used  as  a 
motor;  C  is  a  shunt  wound 
loss  supplier.  The  test 
would  be  easier  with  all  FIGi  I3I 

shunt  fields  separately  ex- 

cited,  but  we  will  suppose  all  machines  to  be  self- 
exciting,  which  will  necessitate  the  use  of  lamp  bank 
or  water  box,  Z,  to  be  used  in  starting,  because: 
C,  a  shunt  machine,  is  used  to  start  the  system,  and, 
since  it  is  self-exciting  the  result  of  closing  K  would  be 
to  have  Close  its  field,  even  assuming  that  it  would  be 
possible  to  lower  C's  voltage  sufficiently  to  safely  close 
K.  C  is  driven  by  the  engine  E;  A  and  B  are  belted  or 
clutched  together.  Let  us  assume,  then,  that  A  and  B 
are  500  volt  500  kilowatt  compound-wound  machines, 
and  that  C  is  a  125  volt  machine  of  the  same  current 
capacity.  A  is  cumulatively  connected  as  a  dynamo, 


412  TESTING    OF    DYNAMOS    AND    MOTORS. 

and  B±  the  same,  as  motor,  /.  <?.,  in  all  cases  their  series 
and  shunt  windings  must  assist  each  other.  It  is  only 
necessary  to  know  the  connection  as  dynamo  when  that 
as  motor  is  secured  by  reversing  the  series  field.  For 
reasons  already  shown,  a  differentially  connected  motor 
will  run  as  a  cumulatively  connected  dynamo  and  vice 
versa.  On  shunt  machines,  for  the  same  direction  of 
rotation  the  connections  are  the  same,  and  separately  ex- 
cited machines  follow  the  same  law.  At  starting  there  is 
no  field  on  A,  and  when  the  time  comes  to  make  its  field 
there  must  be  no  doubt  as  to  its  polarity,  to  avoid  intro- 
ducing it  into  circuit  as  a  motor.  A,  being  self-exciting, 
will  not  pick  up  its  field  till  a  certain  speed  is  reached, 
and  at  that  point  is  apt  to  pick  up  so  rapidly  as  to  give 
trouble  unless  C's  field  is  promptly  strengthened  to  raise 
the  voltage  supplied.  It  must  be  borne  in  mind  that  as 
soon  as  a  machine  generates  voltage  it  begins  to  do  work 
if  the  circuit  is  closed,  and  since  the  machines  are  all  in 
series  and  have  the  same  current,  the  amount  of  work 
that  each  does  is  proportional  to  its  voltage.  The  order 
of  starting  up  is  as  follows:  C  is  first  brought  up  to  full 
speed  by  the  engine,  with  K  open,  to  enable  C  to  get  as 
much  field  as  125  volts  will  give  it,  K\*>  now  closed  and  L 
plugged  till  the  system  gets  well  started.  With  the  lamp 
bank  in,  the  supplier  will  keep  the  system  in  motion  at  a 
fair  rate  of  speed,  until  A  begins  to  pick  up  a  field  and 
do  work,  when  the  demand  on  C  becomes  so  much 
greater  that,  unless  some  of  L  is  cut  out  so  that  C  can 
devote  its  voltage  to  the  motor  instead  of  Z,  the  system 
will  slow  down  until  it  reaches  a  point  where  A  loses  its 
field,  then  the  load  being  off,  the  motor  will  speed  up 
again,  until  A  once  more  acquires  its  field,  only  to  repeat 


COMPOUNDING.  4T3 

the  same  cycle.  The  reason  for  this  action  is  this:  The 
two  machines,  A  and  C,  being  in  series  as  dynamos,  the 
amount  of  electrical  work  each  does  depends  upon 
the  E.  M.  F.  each  generates,  and  this  in  turn  upon  their 
respective  field  strengths;  now  C,  in  its  work,  is  sup- 
ported by  the  engine  to  which  it  is  belted;  but  A  owes 
its  energy  of  rotation  primarily  to  C,  and  then  retains 
it  by  its  own  inertia.  The  result  is  that  when  A 
acquires  its  field,  it  throws  on  the  two  belted  machines 
a  load  entirely  out  of  proportion  to  the  amount 
of  energy  available  from  C,  and  unless  this  amount  is 
increased  accordingly,  the  machines  must  slow  down 
until  the  dropping  of  A's  field  removes  the  abnormal 
load.  If  C  is  separately  excited,  L  need  not  be  used: 
then  C"s  voltage  is  reduced  to  35  or  40  volts  and  K 
closed.  To  facilitate  an  easy  start,  it  is  well  to  have  B^ 
field  circuit  resistance  low  so  it  will  take  as  much  current 
as  possible,  and  better  still  to  start  the  belt  by  hand, 
thereby  raising  the  C.  E.  M.  F.  of  j9's  armature  and 
letting  more  current  through  its  shunt  field.  If,  as  in 
the  present  case,  B  is  compound-wound,  its  series  wind- 
ing gives  it  a  good  starting  torque,  and  the  hand  start  is 
unnecessary.  So  also  by  using  the  lamp  bank,  B,  has 
a  strong  shunt  field  even  before  K  is  closed.  Assuming 
the  system  started,  whatever  method  may  have  been 
adopted,  as  soon  as  a  fair  speed  is  attained,  the  polarity 
of  A  is  tested  with  reference  to  that  of  C.  If  A  is  sepa- 
rately excited  the  test  is  made  by  closing  the  shunt  field 
circuit  through  considerable  resistance,  and  observing 
either  an  ammeter  in  the  main  circuit,  or  a  voltmeter 
placed  across  A  and  C:  if  A  and  Care  in  series,  as  they 
should  be,  making  the  field  on  A  will  increase  the  read- 


414  TESTING    OF    DYNAMOS    AND    MOTORS. 

ing  on  the  voltmeter,  also  that  on  the  ammeter.  Another 
-indication  of  the  same  fact  is  a  sudden  falling  in  the 
speed,  showing  that  the  introduction  of  A  has  increased 
the  load,  while  it  has  not  increased  correspondingly  A's 
ability  to  do  the  added  work.  If  A  and  C  prove  to  be 
opposed,  the  fact  is  indicated  by  a  decrease  in  the  volt- 
meter and  ammeter  reading:  for  since  the  E.  M.  Fs.  are 
opposed,  the  voltmeter  will  register  their  difference;  and 
since  A  and  B  both  are  then  motors,  the  sum  of  their 
C.  E.  M.  Fs.  is  greater  than  that  of  B  alone,  hence  the 
•decrease  in  current  and  speed.  Thus,  we  see  that  the 
speed  decreases  in  both  cases,  but  for  different  reasons. 
If  A  and  Care  found  not  to  be  in  series,  and  C  is  sepa- 
rately excited,  its  shunt  field  must  be  reversed;  if  self- 
exciting,  A's  fields  must  be  charged  from  C,  by  tempo- 
rarily reversing  A's  shunt  winding,  raising  its  brushes, 
-closing  K,  when  the  field  of  A  will  become  so  charged 
as  to  leave  its  residual  field  in  reversed  polarity  to  what 
it  was  before.  A's  connections  are  now  restored,  for  if 
•left  reversed  A  will  not,  for  reasons  to  be  seen  later, 
generate.  Upon  opening  K,  lowering  the  brushes  and 
again  starting  up,  A's  E.  M.  F.  will  be  found  in  series 
with  C's. 

The  system  is  now  in  motion  with  a  slight  load  on, 
but  with  the  speed  low.  The  next  step  is  to  work  on 
full  load  and  adjust  the  speed.  A  and  Care  generators 
in  series,  and  running  B  as  a  motor.  While  this  is  their 
relation  to  each  other,  their  relation  to  the  circuit  is  very 
different.  C,  being  belted  to  £,  maintains  its  speed  inde- 
pendently of  load  variations;  /.  e.,  unless  E  itself  becomes 
overloaded.  Such  is  not  the  case  with  A.  As  its  load 
goes  on,  it  must  depend  upon  B  to  keep  up  its  speed,  and 


COMPOUNDING.  415 

B  in  turn  depends  jointly  upon  A  and  C.  The  first 
effect  of  putting  a  field  on  A  is  to  throw  a  load  on  B, 
whose  speed  will  fall  unless  C's  E.  M.  F.  is  brought  up, 
thus  calling  upon  E  for  support.  The  office  of  C,  then, 
is  to  supply  the  additional  energy  necessary  to  keep  up 
the  speed  as  the  load  is  increased  on  A.  The  load  is 
worked  on  by  slowly  strengthening  A's  field,  with  an  eye 
on  the  ammeter,  and  at  the  same  time  strengthening  C's 
field,  but  weakening  B's.  After  the  usual  run,  A  is  com- 
pounded, and  the  change  over  made.  To  shut  down  the 
test,  A's  and  C's  E.  M.  Fs.  are  lowered,  and  fi's  C.  E. 
M.  F.  raised  by  strengthening  the  field.  When  the  load 
is  nearly  off,  K  is  opened  and  the  system  stopped. 

No  free  data  can  be  taken  in  this  test,  for  A's  armature 
always  carries  a  current,  hence  some  load.  At  the  end 
of  the  test,  however,  an  independent  test  can  be  run, 
using  B  as  a  motor  and  supplying  its  voltage  from  some 
500  volt  machine.  Points  of  particular  care  in  this  test 
for  the  most  part  relate  to  the  putting  on  of  the  load. 
When  it  is  desired  to  make  A  work,  its  field  circuit  resist- 
ance is  slowly  worked  out,  till  the  field  begins  to  pick  up, 
when  the  resistance  should  be  promptly  worked  in 
again,  but  not  to  a  point  where  A  will  lose  its  field. 
The  introduction  of  resistance  should  be  governed  by  the 
indication  of  a  voltmeter  placed  across  A's  terminals. 
As  soon  as  the  field  begins  to  pick  up,  the  needle  will 
rise  rapidly.  Resistance  should  then  be  used  until  the 
needle  stops;  but  if  it  starts  back,  more  resistance  must 
be  cut  out,  otherwise  A  will  drop  its  field.  C's  voltage  is 
increased  each  time  that  the  load  is  increased.  It  must 
be  kept  in  mind  that  A's  E.  M.  F.  controls  the  load,  and 
C's  the  speed.  When  A's  E.  M.  F.  reaches  its  proper 


416  TESTING    OF    DYNAMOS    AND    MOTORS. 

value,  the  rest  of  the  load  is  put  on  by  weakening  .Z?'s 
field.  If  A  is  compound-wound,  however,  its  rheostat  is 
put  at  as  near  as  can  be  reckoned  its  position  for  normal 
voltage,  free,  and  the  load  put  on,  letting  the  series 
winding  bring  the  voltage  up  to  what  it  may. 

If  A  and  C  happen  to  be  opposed  in  E.  M.  F.,  thus 
making  B  and  A  motors  in  series  and  running  from  C,  as 
generator,  the  inertia  of  the  system  will,  if  A's  field  picks 
up  too  rapidly,  enable  their  combined  E.  M.  Fs.  to  run  C 
as  a  motor,  with  the  usual  brush  display.  The  flow  of 
current  in  this  case,  however,  would  be  limited  by  the 
fact  that  j^'s  fields,  cumulatively  connected  as  motor, 
would  act  differentially  as  soon  as  B  became  a  generator. 
If  C  is  compound-wound  the  reversed  current  through 
its  series  windings  may  reverse  its  polarity,  unless  its 
shunt  winding  is  separately  excited,  when  A,  B,  and  C  will 
be  generators  in  series,  working  on  short  circuit.  Such 
a  reversal  is  indicated  by  the  ammeter  needle  falling  to 
zero,  and  then  either  rising  again,  or  deflecting  to  the 
wrong  side,  according  as  the  meter  is  of  an  alternating  cur- 
rent or  a  direct  current  type.  There  will  also  be  a  general 
sparkingat  the  brushes.  This  lasts  only  until  A  and  B  ex- 
pend their  energy  of  inertia,  slow  down,  A  drops  its  field, 
and  the  speed  rises  once  more.  If,  however,  C's  voltage 
is  high,  and  the  reversal  is  so  violent  as  to  bring  the 
system  to  a  sudden  stop,  A  and^  will  start  up  as  motors, 
if  separately  excited.  If,  however,  A  and  B  are  both 
self-exciting,  at  the  instant  when  they  would  become 
motors  neither  has  any  shunt  field,  since  the  low  resist- 
ance armatures  short  circuit  the  shunt  windings,  and  the 
series  fields  tend  to  turn  the  armatures  in  opposite  direc- 
tions, with  the  result  that  they  do  not  turn  at  all,  but 


COMPOUNDING.  417 

stand  at  a  short  circuit  through  C.  If  C  is  self-exciting, 
it  loses  its  field,  or  belt  possibly.  If  compound-wound  the 
series  winding  prevents  its  losing  the  field,  and  if  its 
E.  M.  F.  is  high  enough  the  belt  must  go.  If  C  is  com- 
pound-wound self-exciting,  and  A  and  B  are  separately 
excited,  and  a  too  sudden  strong  increase  in  A's  field 
causes  a  reversal,  C's  polarity  is  reversed  by  the  reversed 
current  in  its  series  windings,  its  shunt  winding  picks  up 
accordingly,  and  if  A  and  B  are  not  already  stopped  by 
the  reversal,  they  will  be,  because  the  polarity  of  their 
fields  is  unchanged,  while  the  E.  M.  F.  impressed  upon 
their  armatures  has  been  reversed.  At  the  risk  of  per- 
haps tiresome  repetition,  we  might  enumerate  many  other 
manifestations  depending  upon  the  type  and  manner  of 
exciting  A,  B  and  C. 

If  necessary  there  is  no  objection  to  pulling  the  line 
switch  under  full  load,  except  that  the  motor  brushes 
may  be  well  back,  and  the  resulting  flashing  injures  the 
commutator.  If  C's  field  gets  broken  under  load,  it 
leaves  A  and  B  to  run  back  on  each  other,  precipitating 
a  heavy  load,  and  stopping  them.  If  C  is  compound- 
wound  the  E.  M.  F.  due  to  the  series  turns  remains  in 
series  with  that  of  A,  helping  to  turn  .#  as  motor.  If  B, 
however,  is  separately  excited,  as  is  often  the  case,  it 
still  has  a  field  after  A's  goes  (due  to  fall  of  speed),  and 
tries  to  run  A  and  C  in  the  opposite  direction  as  motors. 
The  result  is  a  short  circuit  which  brings  A  and  B  to  a 
stand. 

Where  a  low  voltage  machine  of  sufficient  current 
capacity  is  available,  this  test  is  commendable,  and  is 
correct  practice.  Shunt,  compound-wound,  and  sepa- 
rately excited  machines  can  be  run  in  a  motor-generator 


418  TESTING    OF    DYNAMOS    AND    MOTORS. 

test  under  almost  any  conditions,  but  series  machines 
cannot.  The  latter  require  that  the  loss  be  supplied, 
either  by  an  engine,  by  a  machine  in  series  with  the  load, 
or  by  a  machine  having  no  electrical  connection  with 
the  system.  Practically,  series  machines  fall  under  two 
classes:  arc  light  dynamos  and  street  railway  motors. 
Arc  machines  are  always  tested  on  a  lamp  load,  to  realize 
working  conditions.  It  being  impracticable  to  lay  down 
general  rules  covering  all  types  of  series  machine,  the 
three  principal  types  have  been  separately  considered  in 
a  previous  chapter.  Motor  testing  will  be  considered 
in  another  chapter,  and  points  covering  series  machine 
testing  will  be  given  there. 


CHAPTER   XII. 

MISCELLANEOUS    TESTS. 

HAVING  considered  dynamo  tests  generally  adopted  in 
good  practice,  we  will  now  consider  a  variety  of  tests 
which  belong  to  the  experimental  stage  of  development 
of  all  machines.  These  are:  (i)  Core  Loss;  (2)  Satura- 
tion; (3)  Distribution;  (4)  Efficiency,  Electrical  and 
Mechanical,  and  (5)  incidentally,  Hysteresis  and  Fric- 
tion. 

TEST  XII.—  Core  Loss  Test.—  The  test  for  measuring 
the  work  done  in  turning  a  naked  armature  core  in  an 
excited  field  constitutes  a  core  loss  test,  and  this  test  is. 
run  on  every  new  type  of  machine,  and  at  intervals  on  old 
types.  If  the  loss  is  thought  excessive,  a  change  is  made 
in  the  quantity,  quality,  or  disposition  of  the  iron  used. 
To  better  understand  what  is  meant  by  "core  loss," 
we  will  recapitulate  a  little.  Wfcen  a  moving  conductor 
cuts  lines  of  force  an  E.  M.  F.  is  set  up,  and  a  current 
flows  if  the  circuit  be  a  closed  one.  Now,  iron  is  a  con- 
ductor, and  a  solid  armature  body  is  just  as  much  a  closed 
circuit  as  is  a  bare  wire  properly  disposed.  The  old 
Siemens  solid  bodies  are  in  evidence  of  this.  These 
induction  currents  are  greatly  reduced,  but  not  entirely 
eliminated,  by  laminating  the  armature  bodies.  The 
next  source  of  loss  is  "hysteresis,"  which  is  a  mole- 
cular opposition  to  the  magnetizing,  demagnetizing,  andi 


42O  TESTING    OF    DYNAMOS    AND    MOTORS. 

remagnetizing  of  every  part  of  the  core  as  many  times 
per  revolution  as  there  are  pairs  of  poles.  This  "molec- 
ular friction,"  as  it  were,  resists  the  rapid  reversals  of 
polarity  and  manifests  itself  as  heat. 

The  armature  core  under  test  is  set  in  a  frame  and 
belted  to  a  motor,  through  which  is  electrically  measured 
the  work  done  in  the  system.  The  instruments  required 
are  disposed  as  follows:  A  voltmeter  across  the  motor 
armature  and  one  across  its  separately  excited  field;  an 
ammeter  in  the  motor  circuit,  and  one  in  the  field  circuit 
of  the  naked  core  under  test;  a  speed  indicator  or  a 
tachometer.  Of  the  energy  given  the  motor,  part  is 
dissipated  as  I*R  losses  in  motor  armature  and  field,  and 
the  rest  is  expended  in  turning  the  two  armatures  against 
frictional  and  other  opposing  forces.  The  armature  / 2  R 
loss  varies  as  the  field  of  the  core  is  varied,,  but  that  in 
the  motor  field  is  kept  constant  throughout  the  test. 
This  is  accomplished  by  thoroughly  heating  the  fields 
before  the  test,  and  by  keeping  the  applied  E.  M.  F. 
constant  during  the  test:  or  better  to  keep  the  field  cur- 
rent constant  by  means  of  a  field  rheostat  and  low  read- 
ing ammeter.  The  frictional  losses  are  air  fanning, 
motor  brushes,  bearings,  and  belt  tension.  As  the  speed 
is  kept  constant,  frictional  losses  may  be  regarded  as 
constant  also.  The  power  given  to  the  motor  =  im- 
pressed E.  M.  F.  X  armature  current,  =  gross  power 
consumed.  Call  the  motor  A,  and  the  core  with  its 
separately  excited  field,  B.  To  separate  the  friction 
losses  of  A  and  B,  A  is  run  free  at  the  speed  which  would 
be  necessary  to  run  B  at  its  proper  speed,  and  the  power 
measured.  Subtract  from  this  the  7a  R  loss  in  A  and 
A's  bearing  and  brush  friction  loss  is  left.  With  the  belt 


MISCELLANEOUS    TESTS.  421 

on,  the  speed  is  adjusted  and  the  power  again  measured. 
The  difference  between  the  two  measurements  is  due  to 
^'s  bearings  and  the  belt  tension.  In  the  last  case  care 
must  betaken  that  B  has  no  field,  otherwise  the  apparent 
loss  will  be  too  high. 

The  test  proper  now  begins,  and  consists  in  putting 
variable  field  currents  through  J?s  field,  and  noting  the 
power  consumed  by  A  while  running  at  the  proper  speed. 
As  v9's  field  current  increases,  so  do  the  induction  cur- 
rents, etc.,  in  the  core,  and  A  must  be  supplied  with 
more  energy,  or  the  speed  will  fall.  To  facilitate  speed 
regulation  without  disturbing  A's  field,  its  armature  is 
supplied  from  a  variable  source  of  E.  M.  F.,  as,  for 
example,  a  dynamo  whose  field  regulation  affords  a  ready 
means  of  altering  its  E.  M.  F.  For  this  test,  this  means 
of  regulation  has  the  following  advantages:  (i)  There  is 
less  liability  of  exceeding  the  motor's  current  carrying 
capacity  if  the  field  is  left  strong;  (2)  the  field  loss  on 
A  being  kept  constant,  it  does  not  enter  into  the  calcula- 
tions except  as  a  constant,  and  its  effect  on  the  other 
factors  is  always  the  same;  (3)  by  making  the  initial 
field  strong,  the  armature  current  necessary  to  supply 
the  required  work  is  small,  the  impressed  E.  M.  F.  being 
made  correspondingly  great,  and  a  lower  reading  ammeter 
can  be  used,  thus  lessening  the  errors  of  adjustment  and 
observation;  (4)  there  is  freedom  from  sparking;  (5) 
with  a  constant  field,  the  armature  E.  M.  F.  can  be  used 
to  check  up  the  accuracy  of  the  speed  readings.  Should 
there  be  an  abnormal  increment  in  the  voltmeter  reading 
at  any  point,  the  indication  is  that  there  has  been  error 
in  taking  the  speed.  There  is  quite  a  trick  in  handling  a 
tachometer  properly:  in  the  first  place,  it  should  not  be 


422  TESTING    OF    DYNAMOS    AND    MOTORS. 

leaned  against,  because  it  injures  the  instrument  and  adds 
slightly  to  the  load.  It  should  be  held  lightly  and  level. 
To  decrease  the  pressure  necessary  to  keep  it  from 
slipping,  its  point  should  be  covered  with  soft  tape 
so  as  to  fit  the  hole  in  the  shaft,  and  both  hole  and 
point  should  be  well  chalked.  The  tachometer  should 
be  level,  and  tilting  it  either  way  lowers  its  reading, 
because  the  instruments  generally  depend  upon  the 
principle  of  centrifugal  motion,  and  therefore  give  their 
true  and  maximum  reading  when  the  axis  of  rotation  is 
vertical,  and  the  force  of  gravity  properly  directed. 

There  must  be  considerable  range  of  voltage  available, 
and,  if  necessary,  two  machines  in  series  to  supply  it. 
The  instrument  in  the  motor  field,  whether  volt-  or 
ammeter,  need  not  be  very  accurate,  as  it  is  only  required 
to  indicate  its  initial  deflection.  A's  armature  resistance 
must  be  known  so  its  /2  R  loss  can  be  accurately  figured 
for  the  different  current  values.  It  is  best  to  heat  A 
throughout  before  using,  then  its  armature  resistance, 
hot,  can  be  measured  and  can  be  assumed  to  remain 
constant  throughout  the  test.  The  frictional  losses 
having  been  determined,  full  field  is  put  on  B,  the  speed 
adjusted  and  a  power  reading  taken.  Subtracting  from 
this  the  frictional  loss,  and  the  /3  R  loss  in  A,  the  core 
loss  remains,  £'s  field  current  is  then  decreased  by 
regular  steps,  till  zero  is  reached,  when  it  is  reversed 
and  increased  in  the  same  manner.  The  results  should 
be  tabulated  in  parallel  columns,  and  the  core  loss  deter- 
mined for  each  current  value.  A  curve  is  then  plotted, 
in  which  the  horizontal  scale  gives  the  current  values, 
the  vertical  scale  the  corresponding  core  losses.  The 
readings  are  started  with  full  field  on  JE>,  to  insure  that 


MISCELLANEOUS    TESTS.  423 

the  facilities  at  hand  are  adequate  to  maintain  the  speed 
for  all  loads  without  excessively  overloading  A  or  exceed- 
ing the  ammeter's  range.  This  test  when  interrupted 
cannot  be  resumed  where  the  readings  left  off,  but  must 
be  run  over  again  in  order  that  the  influence  of  the 
residual  field  may  be  uniform  throughout.  The  above 
method  secures  a  regular  variation  from  full  positive  field 
to  zero,  back  to  full  negative  field.  If  the  current  is 
carried  in  the  same  steps  back  to  the  starting  point,  we 
complete  a  cycle  of  magnetization. 

TEST  XIII.  —  Eddy  Current  Test.  —  The  above  test  sepa- 
rates the  electrical,  magnetic,  and  mechanical  losses,  but 
gives  no  detailed  information  in  regard  to  hysteresis, 
or  to  eddy  currents.  To  determine  these,  it  is  neces- 
sary to  run  the  completed  armature  as  a  separately 
excited  motor  at  at  least  two  different  speeds.  The 
losses  due  to  friction  and  to  hysteresis  vary  (approxi- 
mately) directly  as  the  speed;  /.  <?.,  if  the  speed  is 
doubled  the  losses  are  doubled.  The  loss  due  to 
foucault  or  eddy  currents  varies  as  the  square  of  the 
speed;  /".  e.,  if  the  speed  is  doubled,  the  loss  is  quad- 
rupled. Calling  the  total  loss  JF,  it  is  divided  as 
follows:  friction  Wv\  hysteresis  £FH,  eddy  currents  JFE, 
and  we  may  write, 


The  first  two  were  determined  in  Test  XII,  so  that 
they  are  known  quantities.  If  now,  with  the  same 
field  excitation  the  voltage  is  raised  till  the  speed  is 
doubled,  we  have  the  following  equation: 


424  TESTING    OF    DYNAMOS    AND    MOTORS. 

Combining  this  with  equation  (i),  we  get, 

W'  -  2  W  =  2  W^   ............  (3) 

or 

Wz=~-  W, 

which  gives  the  eddy  current  loss  without  knowing  the 
friction  loss  or  hysteresis  loss,  but  knowing  their  com- 
bined losses,  W.  The  above  expression  supposes  the 
second  speed  to  be  double  the  first;  this  is  not  a  neces- 
sary condition.  Let  us  suppose  the  new  speed  to  be  K 
times  the  old  speed,  where  K  is  either  a  whole  number 
or  a  fraction,  Eq.  (2)  then  becomes, 


W  =  KWW  +  KWn  +  K*Wm  .....  (4) 
Eq.  (i)  is  W  =  Ww.+  Wn  +  Wz: 

multiplying  Eq.  (i)  by  K,  we  get 

KW  =  KWY  +  KW*  +  KW^  .......  (5) 

subtracting  Eq.  (5)  from  Eq.  (4)  we  get 

W  -  KW  =  K^W^-KW*  .........  (6) 

or 

W    -  KW-  KW*(K  -  i), 
or 

w  _W'-KW 

W*       K(K  -  i)' 

Then  suppose  K  =  2.      Then, 


as  above. 


MISCELLANEOUS    TESTS.  425 

Let  K  =  3/2,  then 

,FE  =  iv'  -  3/*jr  - 


3/2  (3/2  -  i)  3/4 

80"     -  12 


In  all   cases   several    speeds   should   be   taken   and    the 
values  for  JFK  compared. 

In  core  loss  tests  run  under  full  load,  as  they  often  are, 
it  is  desirable  to  know  how  much  power  is  consumed  in 
driving  the  system  when  the  generator  field  is  very  weak 
and  there  is  full  current  in  the  armature.  Under  these 
conditions  the  brushes  are  brought  well  forward  to 
eliminate  the  sparking,  and  the  armature  reaction  is  so 
great  that  there  is  liability  to  reversal.  If  the  machine 
is  shunt-wound,  it  drops  its  field  when  reversal  takes 
place,  or  even  before,  because  greatly  increasing  the 
field  circuit  resistance  has  the  same  effect  as  greatly 
decreasing  the  line  resistance.  If  compound-wound  or 
separately  excited,  it  cannot  do  this.  On  a  compound- 
wound  machine  the  field  picks  up  with  the  reversed 
polarity.  On  a  separately  excited  machine  the  polarity 
cannot  thus  be  permanently  reversed,  but  is  over- 
powered for  the  time  being.  If  the  above  investigation 
is  the  only  object  of  the  test,  it  may  be  more  convenient 
to  run  the  generator  back  on  a  motor,  and  supply  the 
loss  from  an  engine  belted  to  both.  In  this  case  the 
result  of  a  reversal  is  to  throw  the  two  machines  in  series 
as  generators,  with  a  short  circuit  through  the  two 
armatures.  An  ammeter  in  the  dynamo  self-excited 
shunt-field  circuit  will  be  found  to  have  reversed  its 


426  TESTING    OF    DYNAMOS    AND    MOTORS. 

deflection  after  the  field  has  picked  up  to  suit  the  new 
conditions. 

TEST  XIV.—  Saturation  Test. — The  next  test  to  be 
considered  is  the  saturation  test,  so  called  because 
it  shows  when  the  field  cores  are  magnetically  satu- 
rated. This  test  is  usually,  though  not  necessarily, 
run  in  conjunction  with  the  core  loss  test,  and 
determines  the  extent  to  which  it  is  profitable  to 
expend  energy  in  magnetizing  the  fields  of  a  dynamo. 
When  a  machine  under  test  develops  a  lower  E.  M.  F. 
than  experience  with  that  type  leads  one  to  expect,  the 
natural  inference  is  that  an  inferior  quality  of  iron  has 
found  its  way  into  the  frame.  A  saturation  curve  of  this 
machine,  when  compared  with  those  of  other  machines, 
will  show  whether  this  inference  be  true.  The  test  is 
run  with  the  assumption  that  the  more  common  sources 
of  error,  connections,  loose  joints,  windings,  crosses,  etc., 
have  been  eliminated.  The  test  consists  in  running  the 
armature  free  at  proper  speed,  and  in  separately  excited 
fields.  An  ammeter  is  placed  in  the  field  circuit,  and  a 
voltmeter  across  the  brushes.  The  field  current  is  grad- 
ually increased  from  zero  to  a  point  where  for  a  given 
increase  in  field  current  the  increase  in  E.  M.  F.  is  about 
constant.  As  the  field  current  is  increased  step  by  step, 
the  increase  in  E.  M.  -F.  is  also  recorded,  and  a  curve 
plotted,  with  field  current  values  as  abscissas  and  E.  M.  Fs. 
as  ordinates.  Suppose  five  amperes  to  be  the  limit  to 
which  it  is  safe  to  temporarily  carry  the  field  current: 
commencing  with  zero  field  current,  the  E.  M.  F.  due  to 
residual  field  is  read,  and  then  the  field  current  increased 
in  steps  of  one-fourth  ampere,  and  the  E.  M.  F.  read  at 
each  step.  The  E.  M.  F.  will  be  found  to  increase  rapidly 


MISCELLANEOUS    TESTS.  427 

at  first,  and  finally  by  equal  amounts.  Having  reached 
the  upper  limit  the  current  is  decreased  by  the  same  steps 
until  zero  is  reached,  when  the  current  is  reversed  and 
the  operation  repeated,  thus  completing  the  cycle.  It  is 
important  that  the  field  current  should  not  be  broken 
throughout  the  test,  save  at  the  zero  readings;  otherwise 
the  curve  will  not  be  a  smooth  one,  and  the  test  must  be 
repeated.  For  a  similar  reason  care  must  be  taken  not 
to  exceed  a  current  value  in  passing  to  it,  and  if  it  is  ex- 
ceeded not  to  return  to  it,  but  note  its  actual  value  and 
pass  to  the  next.  The  reason  for  this  is  that  as  the  cur- 
rent increases  the  iron  tends  to  hold  its  magnetization, 
and  if  the  current  be  interrupted  this  retaining  power 
will  not  immediately  resume  its  former  value.  In  reced- 
ing to  a  reading,  the  reading  is  likewise  influenced  by  the 
higher  degree  of  magnetization  due  to  the  current  receded 
from.  This  retaining  power  is  illustrated  by  the  differ- 
ence between  the  up  and  down  readings  of  voltage,  the 
latter  of  which  for  the  same  current  are  always  higher 
than  the  former.  In  this  test  it  is  all  important  that  the 
speed  be  maintained  constant,  and  that  the  volt  lines  have 
good  contact. 

The  rapid  increase  of  E.  M.  F.  at  first  shows  the  iron 
to  be  far  from  saturation,  but  as  the  "  knee  "  of  the  curve 
is  reached  the  iron  becomes  more  highly  magnetized, 
and  just  around  the  knee  is  said  to  be  saturated. 
Beyond  this  point  the  increase  in  magnetization  is  not  at 
all  proportional  to  the  increase  in  magnetizing  current, 
and  it  is  uneconomical  to  use  a  larger  current  than  that 
corresponding  to  the  point  just  beyond  the  knee. 
When  the  curve  is  plotted  it  will  be  found  that  the 
descending  curve  lies  above  the  ascending  one  and  does 


428  TESTING    OF    DYNAMOS    AND    MOTORS. 

not  cross  the  zero  of  E.  M.  F.  till  the  current  has  been 
reversed.  The  descending  reading  of  E.  M.  F.,  taken 
when  the  current  is  zero,  is  a  measure  of  the  retentive 
power  of  the  iron,  and  may  be  called  its  temporary  field. 
If  left  alone  for  some  time  this  effect  disappears,  and  the 
residual  magnetism  resumes  its  original  value.  The 
reversed  current  necessary  to  reduce  the  E.  M.  F.  to 
zero  before  it  changes  sign,  is  a  measure  of  the  iron's 
coercive  force,  and  is  but  another  way  of  measuring  the 
retentive  power.  All  instruments  should  be  calibrated 
immediately  before  the  test,  and  should  be  provided  with 
push  buttons  or  cut-out  switches  to  relieve  them  when 
not  being  read,  and  thus  minimize  the  effect  of  tempera- 
ture variation.  Instruments  should  not  be  near  enough 
to  influence  each  other,  and  should  be  removed  as  much 
as  possible  from  external  magnetic  influences.  The  test 
for  such  errors  is  to  turn  the  instrument  around  and 
observe  if  its  own  or  any  other  instrument's  deflection  i& 
changed.  They  must  be  calibrated  after  test  to  insure 
that  no  jar  or  exposure  has  introduced  an  error.  Once 
a  test  is  started  all  existing  conditions  must  remain  the 
same;  even  those  which  might  have  been  corrected  at 
first  must  now  remain  undisturbed.  Oil  must  not  be  used 
on  the  commutator  of  the  dynamo,  as  its  variable  resist- 
ance will  cause  a  marked  difference  in  the  voltmeter 
readings.  Very  often  it  is  required,  after  the  free  satu- 
ration test  above,  to  run  one  with  variable  currents  in  the 
armature:  by  comparing  the  two  the  effect  of  armature 
reaction  is  readily  seen.  Finally,  if  for  any  reason  the 
test  is  shut  down  before  completion,  the  partial  data  is 
of  no  value,  and  the  whole  test  must  be  run  over. 

TEST  XV .—Distribution   Test.—  This  test  is   related  to 


MISCELLANEOUS    TESTS.  429 

Test  XIV.  in  that  both  have  to  do  with  the  magnetic  field. 
Its  object  is  to  determine  the  character  and  strength  of 
the  field  all  around  the  commutator;  to  determine  the 
field's  variation  from  point  to  point,  and  whether  this 
variation  is  such  as  would  be  expected  or  could  be 
improved  by  altering  any  of  the  details  of  construction. 
The  test  is  run  first  with  the  armature  free,  so  as  to 
determine  the  normal  character  of  the  field,  then  at  in- 
termediate and  full  load, 
to  determine  the  law  of  in- 
crease  and  the  full  effect 
of  the  armature  reaction. 
There  are  two  principal 
methods,  either  of  which 
may  be  adopted:  i.  One 
terminal  of  a  voltmeter  is  FIG.  132. 

fastened    to   one    of    the 

brushes,  the  other  terminal,  attached  to  an  exploring 
brush,  is  moved  around  the  commutator,  bar  by  bar,  till 
it  reaches  a  brush  of  opposite  sign.  This  is  known  as 
Mordey's  method,  and  its  connections  are  given  in  Fig. 
132.  A  is  the  positive,  B  the  negative  machine  brush, 
and  C  the  explorer.  V  is  a  voltmeter,  one  of  whose 
terminals  is  secured  to  A,  the  other  to  C.  Starting  with 
A  and  C  on  the  same  bar,  C  is  worked  bar  by  bar  toward 
B.  At  the  start  no  deflection  obtains  unless  Vis  very 
sensitive,  and  on  some  machines  the  explorer  includes 
several  bars  before  a  readable  deflection  is  obtained.  As 
C  progresses  toward  B,  the  deflection  increases,  and 
upon  reaching  Bt  should  give  the  full  voltage  of  the 
machine.  When  running  the  test  under  load,  two  volt- 
meters may  be  used;  one,  V,  used  with  the  explorer,  the 


430  TESTING    OF    DYNAMOS    AND    MOTORS. 

other,  V't  being  permanently  connected  across  A  and  B. 
V  will,  under  load,  show  a  higher  maximum  deflection 
than  V ,  because  the  latter  does  not  include  the  IR  drop 
due  to  brush  contact.  After  the  "flat"  or  neutral  field 
is  passed,  the  deflection  increases  slowly,  then  more 
rapidly,  and  reaches  its  fastest  rate  of  increase  at  that 
point  on  the  commutator  which  introduces  the  most 
active  coil.  After  this  point  is  passed  the  deflection 
continues  to  increase,  but  at  a  decreasing  rate,  till,  when  C 
approaches  B,  the  inclusion  of  an  additional  bar  does 
not  augment  the  voltage  at  all.  A  double  scale  voltmeter 
facilitates  the  reading  of  the  lower  variations  near  the 
brushes.  It  is  well  to  move  V's  fixed  terminal  to  B,  and 
explore  in  the  reverse  direction.  Upon  exploring  under 
load,  it  will  be  found  that  distortion  has  taken  place,  and 
that  the  field  is  no  longer  symmetrical,  but  is  massed 
toward  the  forward  brush.  This  distortion  is  sometimes 
so  very  great  that  it  is  impossible  to  find  a  strictly  non- 
sparking  point  for  the  brushes.  This  means  that  the 
lines  of  force  are  so  crooked  that  there  is  no  point  at 
which  the  conductors  slide  along  them  without  cutting 
them.  On  one  machine  sold  on  the  market,  Mordey 
found  that  at  one  point  the  inclusion  of  an  additional 
bar  lowered  the  reading,  showing  that  the  E.  M.  F. 
generated  in  that  part  of  the  field  opposed  that  in  the 
other  part. 

To  facilitate  the  movement  from  bar  to  bar,  a  rack  or 
frame  is  made  for  C  to  slide  in,  and  the  face  of  the  rack 
is  graduated  into  division  equal  to  the  width  between 
the  centres  of  consecutive  bars.  By  means  of  a  second 
voltmeter,  V' ,  connected  across  A  and  B,  the  machine's 
voltage  is  kept  constant  during  the  test.  Even  with  the 


MISCELLANEOUS    TESTS. 


431 


best  of  the  care,  F's  needle  will  vibrate  more  or  less, 
due  to  the  variable  contact  resistance  of  the  small 
brush. 

To  obviate  this  inconvenience,  Swinburne's  modifica- 
tion of  Mordey's  method  maybe  used.  Connections  are 
as  in  Fig.  133.  A  and  B  are  the  machine's  brushes;  C,  the 
explorer's;  A  C  B  the  part 
of  the  commutator  included 
between  brushes  of  opposite 
sign;  G  is  a  galvanometer 
whose  constants  need  not  be 
known;  R,  a  resistance  box 
used  to  protect  G  and  to 
vary  its  sensibility;  /and  H 
are  resistance  boxes,  either 
of  which  can  alone  stand 
the  machine's  brush  voltage; 
Vl  and  V^  are  voltmeters  reading  the  potential  differ- 
ence across  /  and  H  respectively,  while  1\  is  another 
for  maintaining  the  machine's  E.  M.  F.  constant. 
The  adjustment  for  taking  readings  is  based  on  the 
following  fact:  brush  A  is  positive,  /?,  negative,  so 
that  in  passing  from  A  to  B,  whether  we  go  through 
the  commutator  A  C  B,  or  the  outside  circuit  AMI 
H  N  B,  we  must  pass  through  a  point  of  zero  potential, 
/.  e.,  in  every  circuit  there  is  an  internal  and  an  exter- 
nal point  which  are  at  the  same  potential  and  a  galvan- 
ometer joining  two  such  points  would  not  be  deflected. 
The  procedure  is  as  follows :  When  C  is  at  A,  Cand  Z>,  the 
two  ends  of  G,  are  at  the  same  potential,  provided  there 
is  no  resistance  in  box  /,  so  that  no  current  flows  through 
G.  V^  then  reads  the  same  as  Vf  As  C  is  moved  toward 


FIG.  133 


432  TESTING    OF    DYNAMOS    AND    MOTORS. 

B,  G's  terminals  are  at  different  potentials,  and  G  deflects 
upon  pressing  its  key.  Such  resistance  is  then  put  in  / 
as  will  make  the  drop  across  it  the  same  as  that  from  A 
to  C.  Pressing  the  key  will  then  no  longer  cause  a 
deflection,  because  (7  and  .D  will  -be  at  the  same  potential. 
In  this  way  V^  reads  the  increasing,  F2  the  decreasing, 
voltage,  while  their  sum  equals  the  reading  of  Vz. 

Care  must  be  taken  that  the  combined  resistance  of 
/  and  H  is  not  so  much  reduced  as  to  cause  an  injuri- 
ous flow  of  current:  the  adjustment  is  effected  not  by 
decreasing  the  total  resistance  of  the  two  boxes,  but 
by  shifting  resistance  from  one  to  the  other,  the  total 
remaining  the  same. 

2.  The  second  method  of  exploring  the  magnetic  field  is 
due  to  Professor  S.  P.  Thompson,  and  consists  in  attaching 

the  voltmeter  terminals 

^^  ^"\^  to  two  brushes,  held  far 

A^ -^B  enough  apart  to  span 

FIG.  134.  the  insulation  between 

adjacent  bars.  Mov- 
ing these  brushes  from  bar  to  bar,  the  voltage  of 
each  single  coil  is  measured,  and  the  sum  -of  these 
voltages  gives  that  of  the  machine.  Any  marked  dis- 
tortion is  thus  readily  detected.  This  method  has  the 
advantage  that  it  gives  directly  the  result  sought,  while 
the  other  requires  more  or  less  calculation.  The  results 
of  a  distribution  test  can  be  graphically  shown  by  letting 
the  horizontal  scale  represent  the  distance  from  the 
positive  to  the  negative  brush,  and  the  vertical  scale  the 
E.  M.  F.  generated  at  each  point.  The  curve  will  then 
rise  to  a  maximum  and  fall  to  zero.  Or  the  vertical 
scale  may  represent  the  total  E.  M.  F.  from  one  brush 


MISCELLANEOUS    TESTS. 


433 


to  the  given  bar;  in  this  case,  the  curve  will  always  rise, 
but  more  rapidly  in  some  places  than  in  others.  Still 
another  way  is  to  represent  the  armature  by  a  circle,  and 
to  draw  the  curve  of  E. 
M.  F.  around  it,  consider- 
ing the  circumference  of 
the  circle  as  a  base  line,  A- 


FIG.  135. 


and  the  vertical  scale  as 

radial    to    it.      Figs.     134, 

*35»   J36  illustrate  these  methods.     For  more  extended 

information,  the  reader  is  referred  to  S.  P.  Thompson's 

Dynamo- Electric  Machinery. 

TEST  XVI. — Brush  Test, — One  other  test,  simple  to 
execute,  and  quite  necessary,  is  the  brush  test,  to  determine 
what  thickness  of  brush  will,  on  a  given  style  of  machine, 
spark  least:  also  what  cross-section  is  necessary  to  carry 
the  required  current  without  undue  heating.  The  brush 
thickness  best  adapted  to  a  given  machine  is  determined 
experimentally  in  each  case.  Some  of  the  factors 
modifying  the  thickness  are:  speed,  field 
strength,  width  of  pole  pieces  and  number 
of  turns  of  wire  per  armature  section,  all 
of  which  affect  more  or  less  the  voltage 
generated  in  the  sections  short  circuited 
by  the  brushes. 

TEST  XVII.—  Efficiency  Test.—  Having 
considered  in  turn  the  several  methods 
of  conducting  temperature  tests,  the 
compounding,  core  loss,  saturation,  distribution,  and 
brush  test,  there  follows  very  naturally  the  efficiency 
test,  as  conducted  on  machines  before  they  were  put  on 
the  market.  The  term  efficiency  is  used  in  two  senses: 


434  TESTING    OF    DYNAMOS    AND    MOTORS. 

1.  The  electrical  efficiency,  a  term  having  to  do  with  the 
machine    considered    purely    as    an    electrical    device; 

2.  Commercial  efficiency,  having  to  do  with  the  machine 
considered    as    an     energy     transforming    device,    and 
expressed    by   the    fraction    obtained    by    dividing   the 
machine's  available   output  by   its   actual    intake.     If  an 
engine  delivers  5,000  foot-pounds  of  work  per  second  to 
the    dynamo's   pulley,  and    only   3,000    foot-pounds   are 
available  for  use  in  the  working  circuit,  the  commercial 
efficiency  is 


5,000 

a  rather  low  figure.  The  other  40  %  has  been  consumed 
in  electrical,  magnetical,  and  frictional  losses.  It  is 
possible,  however,  to  determine  exactly  how  much  elec- 
trical energy  is  produced  in  the  dynamo,  and  also  what 
proportion  of  this  is  delivered  to  the  line,  and  what  pro- 
portion is  wasted  in  the  machine.  The  quotient  arising 
from  dividing  the  energy  delivered  to  the  line  by  the 
total  amount  produced  in  the  electrical  circuit  con- 
stitutes the  electrical  efficiency.  This  ignores  the  iron 
losses  and  frictional  losses,  and  is  therefore  greater  than 
the  commercial  efficiency.  In  the  above  case,  suppose 
4,000  foot-pounds  per  second  of  electrical  energy  is  pro- 
duced, and  3,000  are  delivered  for  use.  The  electrical 
efficiency  is  then 

-  75  *. 


4,000 


It  is  customary  to  express  electrical  energy  in  electrical 
units,  and  since  one  foot-pound  per  second  =  1.3  watts, 
the  total  watts  would  be,  5,200;  watts  delivered  =  3,900. 


MISCELLANEOUS    TESTS.  435 

Electrical  efficiency  -  =  75  % 

as  before.  The  commercial  efficiency  is  the  item  which 
interests  purchasers,  for  the  electrical  efficiency  is  often 
misleading,  inasmuch  as  it  is  sometimes  advantageous  to 
sacrifice  electrical  efficiency  to  gain  some  other  good 
point  more  requisite.  A  direct  driven  dynamo  may  have 
an  electrical  efficiency  of  95  %,  and  its  commercial  effi- 
ciency fall  as  low  as  75$.  There  is  still  a  third  efficiency, 
which  for  want  of  a  better  name  we  will  call  the  "coal 
pile  efficiency,"  and  this  involves  directly  the  amount 
of  coal  used  per  watt-hour  of  electric  energy  available. 
It  is  not  until  this  "  over-all  "  or  "  plant  "  efficiency  is 
raised  to  its  highest  value  that  electrical  stocks  will 
yield  dividends  commensurate  with  the  theoretical 
advantages  of  electrical  transmission. 

To  return  to  the  question  in  hand:  to  determine  the 
commercial  efficiency,  we  must  carefully  measure  the 
intake  and  output  of  the  machine.  The  last  divided  by 
the  first  gives  the  expression  sought.  There  are  several 
mechanical,  and  as  many  electrical,  methods  of  doing  this. 
In  testing-room  practice  it  is  customary  to  run  the 
machines  as  in  Test  X,  where  the  loss  is  supplied 
from  a  dynamo.  Fig.  137  shows  the  connection  of  the 
machines  and  the  disposal  of  the  instruments.  A  is  a 
dynamo  whose  efficiency  is  to  be  determined;  B  is  a 
motor  on  which  A  is  to  run  back,  and  whose  voltage 
must  be  the  same  as  A's-,  C  is  the  loss  supplier.  A  and  C 
each  have  an  ammeter:  their  combined  currents  is  that 
of  B.  To  obtain  greater  accuracy,  a  standard  shunt  and 
galvanometer  can  be  conveniently  placed  for  checking 


436 


TESTING    OF    DYNAMOS    AND    MOTORS. 


up  the  readings  of  both  ammeters.  A  voltmeter  is 
arranged  to  be  thrown  across  the  terminals  of  any 
machine,  or  better  still,  three  voltmeters  are  used.  As 
a  preliminary  step  determine  the  internal  resistances  of 
A  and  B.  This  is  done  by  either  blocking  the 


pulleys  or  reversing  half  the  fields  on  a  machine, 
so  that  it  cannot  start,  and  then  sending  a  current 
through  it  by  means  of  C,  and  reading  the  drop 
on  the  respective  terminals.  The  load  is  next  put 
on  as  in  Test  X,  and  the  speed  and  voltage  of  A 
adjusted.  Calling  JS,  A's  E.  M.  F. ,  and  /its  current,  its 
useful  output  is  E  x  /,  which  gives  the  numerator  of  the 
fraction  expressing  either  the  electrical  or  commercial 
efficiency.  The  next  step  is  to  measure  the  total  energy 
supplied  to  A  mechanically  by  B.  To  do  this  it  is 
necessary  to  measure  the  energy  delivered  to  B,  and  to 
subtract  from  this  the  energy  lost  in  B  itself:  this 
necessitates  running  a  core  loss  test  on  B,  and  at  the 
same  voltage,  current,  and  speed  as  it  has  in  the 
efficiency  test,  otherwise  the  magnetic  and  frictional 
losses  on  the  two  machines  cannot  be  separated.  /?'s 
energy  is  from  two  sources,  A  and  C.  That  furnished 
by  A  is  A's  useful  output  less  the  loss  in  the  connecting 


MISCELLANEOUS    TESTS.  437 

cables— this  cable  loss  is  the  drop  in  the  cables  (in  this 
case  equal  to  the  difference  in  readings  of  VM^  and 
F  J/J  multiplied  by  the  current  /flowing  in  them.  The 
energy  furnished  by  C  is  the  voltage  registered  on  V Mv 
X  C's  current:  or  \V  —  E'  i.  Also  we  have  the  energy 
from  .4  =  (  E—  cable  drop) x/,  or  JF=  (E  -  d)  /.  Then 
W  +  W  =  E'  i  +  E  I  -  d  I  =  E '  i  +  (E  -  d}  I;  i.  e.t 
the  total  energy  given  to  B  —  W  -f-  IV  =  E'  i  -j- 
(E  -  d)  /,  or  JFT  =  E'  i  -f  (E  -  d)  I.  From  this 
expression  we  must  deduct  whatever  work  is  lost  or 
expended  in  the  motor  itself.  To  follow  the  steps  more 
clearly  we  give  a  form  of  test  sheet  with  actual  figures 
appended.  We  assume  all  resistances  and  core  losses, 
etc.,  to  have  been  preliminarily  determined.  Internal 
resistances  are  measured  between  the  machines'  termi- 
nals so  as  to  give  the  multiple  resistance  of  fields  and 
armature.  If  compound-wound,  the  shunt  winding  is 
connected  "long  shunt."  All  of  the  machines  are 
self-excited. 

r  =  A's  internal  resistance  =  .015  ohm. 

/=  A's  current  =       1,000  amps. 

r'=  B's  internal  resistance  =  .015  ohm. 
/   =  C's  current  200  amps. 

I  -\-  i  —  B's  current  =       1,200  amps. 
E  =  A's  voltage  (FJ/J  500  volts. 

E'  =  ^'s  voltage    (FJ/J  498      " 

d-  Cable  drop  ( FJ/, -  FJ/J  =  2     " 

W=  A's  useful  output 

=  500  X  1,000  =  500,000  watts. 
W'  =  Energy  from  C 

=   498    X    200  =      99,600         " 


438  TESTING    OF    DYNAMOS    AND    MOTORS. 

W  -\-  W  =  Total  energy  furnished 

to  B.  =  599,600  watts. 

w  =  Core  loss,  etc.,  in  B  =  30,000       " 

W-\-W  —  w  =  Energy  expended  on  A  =  569,600      " 

A's  commercial  efficiency, 
W 


W  +  W  -  w       569,600 

The  electrical  efficiency  =  useful  electrical  output 
divided  by  total  electrical  output.  /  —  A's  current  — 
1,000  amperes;  r  =  A's  resistance  =  .015  ohms.  /*  r 
=  1,000  X  1,000  —  .015  =  A's  internal  loss  (electrical) 
—  15,000  watts.  W  +  72  r  =  A's  total  production  = 
515,000  watts. 

W 


the  electrical  efficiency. 


CHAPTER   XIII. 

GROUNDS    ON    THE    LINE. 

THE  most  formidable  difficulty  with  which  engineers 
have  to  contend  is  the  maintenance  of  good  line  insula- 
tion. Upon  the  effectiveness  of  this  insulation  depends 
the  smooth  running  of  the  station,  and  in  a  measure  its 
dividends.  However  carefully  the  work  of  installment 
may  have  been  done,  and  however  efficient  may  be  the 
daily  attendance,  grounds  and  crosses  are  always  liable 
to  occur,  and  generally  do  so  at  most  inconvenient  times. 

In  the  present  chapter  it  is  the  writers'  aim  to  give  the 
more  common  and  efficient  methods  of  detecting  faults, 
and  the  more  practical  methods  of  removing  them.  To 
detect  a  ground  is  the  office  of  a  device  known  as  a 
"  ground-detector."  The  method  of  locating  and  remov- 
ing a  ground  depends  upon  its  nature  and  gravity. 

The  presence  of  a  single  ground  in  no  way  impairs  the 
service  of  a  metallic  circuit;  if  one  point  of  a  perfectly 
insulated  circuit  is  earthed,  there  is  a  momentary  flow 
of  current  which  causes  a  re-distribution  of  potential 
throughout  the  system,  and  brings  the  point  in  question 
to  the  earth's  potential,  whereupon  the  equalizing  current 
ceases  to  flow.  In  many  cases  the  ground  detector  itself 
constitutes  the  harmless  ground,  and  depends  for  action 
upon  the  occurrence  of  a  second  ground  elsewhere  on 

439 


440  TESTING    OF    DYNAMOS    AND    MOTORS. 

the  system.  Fig.  138  gives  the  connections  of  a  common 
form  of  detector.  A  and  B  are  the  two  mains  of  a  two- 
wire  metallic  circuit;  Z,  Z',  two  incandescent  lamps  in 
series  across  them.  L  and  L  are  as  nearly  as  possible 
of  the  same  resistance,  and  under 
ordinary  conditions  burn  at  the 
same  brilliancy.  From  the  junc- 
tion C  of  the  two  lamps  passes  to 
earth  a  wire  C,  S,  G,  including  a 
low  resistance  bell.  Ordinarily 
C  and  G  are  at  the  same  po- 
tential, and  no  current  flows  between  them;  if,  how- 
ever, a  ground  occur,  as  at  Gf,  C's  potential  becomes 
different  from  that  of  the  earth,  a  current  flows  through 
G  C  and  operates  the  signal  bell,  continuing  to  do  so  until 
the  detector  circuit  is  opened.  The  bell  rings,  and  one 
lamp  burns  brighter  than  the  other.  Which  lamp  burns 
brighter  depends  upon  which  side  of  the  line  contains  the 
fault.  If  it  is  on  A,  L  brightens,  but  if  on  B,  L  does  so, 
and  L  grows  dimmer,  because  the  second  ground  at  G' 
places  the  fault  in  multiple  with  Z',  and  the  latter  does 
not  get  as  much  current  as  it  would  if  no  ground  existed. 
If  G'  is  a  ''dead  ground,"  Z'  is  short  circuited  and  Z 
gets  the  entire  line  voltage,  because  Z's  terminals  are  in 
multiple  with  those  of  G',  whose  resistance  is  practically 
zero,  and  since  the  potential  difference  between  A  and 
B  distributes  itself  according  to  the  resistance,  no  drop 
will  take  place  between  the  terminals  of  Z',  and  Z  is  sub- 
jected to  the  entire  line  voltage.  For  this  reason  each 
lamp  should  be  of  the  full  line  voltage. 

The  effect  of  an   external  ground,  then,  is  to  place  in 
multiple  with  one  lamp  a  by-path  whose  shunting  power 


GROUNDS    ON    THE    LINE.  441 

depends  upon  the  extent  of  the  fault;  a  slight  ground 
dimming  the  lamp  with  which  it  is  in  multiple,  a  dead 
ground  extinguishing  it  altogether. 

According  as  A  or  B  is  faulty  the  current  in  C,  S,  G 
will  be  toward  the  earth  or  from  it,  and  depending  upon 
this  fact,  a  pair  of  high  resistance  electromagnets  actu- 
ating a  needle  over  a  graduated  scale,  or  a  pair  of  volt- 
meters, can  replace  the  lamps  and  indicate  the  faulty 
main,  also  giving  a  fair  idea  of  how  serious  the  fault  is. 
If  the  ground  wire  C,  S,  G,  contain  a  voltmeter  adjusted 
to  read  right  and  left  from  a  zero  in  the  centre  of  the 
scale,  or  if  it  contain  an  electromagnet  operating  a  needle 
free  to  move  either  way  from  a  central  position  corre- 
sponding to  zero  current,  connections  are  simplified. 
The  objection  to  such  detectors  is  that  they  sometimes 
give  misleading  indications.  For  instance,  suppose  that 
ordinarily  one  lamp  burns  a  little  brighter  than  the  other, 
indicating  one  main  to  contain  a  slight  ground:  and  that 
upon  a  certain  day  the  discrepancy  disappeared,  the 
lamps  then  burning  at  the  same  brilliancy.  This 
behavior  would  indicate  either  of  two  things,  that  the 
existing  ground  had  disappeared  or  that  a  similar  ground 
had  developed  on  the  other  main.  In  the  first  case  the 
lamps  would  burn  alike  because  neither  would  be  shunted 
by  a  fault,  and  in  the  second  case  they  would  burn  alike 
because  they  would  be  shunted  by  faults  of  equal  resist- 
ance; in  other  words,  a  fault  on  A  would  tend  to  send 
through  (7,  S,  G,  a  current  from  the  earth,  while  a  similar 
fault  on  B  would  have  an  equal  tendency  to  send  through 
C,  6",  G,  a  current  toward  the  earth,  with  the  result  that 
no  current  at  all  would  flow  through  C,  S,  G,  and  the 
lamps  would  burn  as  if  no  ground  existed.  To  test  the 


442  TESTING    OF    DYNAMOS    AND    MOTORS. 

true  condition  of  affairs  we  must  disconnect  the  ground 
wire,  and  by  means  of  a  voltmeter  or  ammeter,  one  of 
whose  terminals  is  earthed  as  at  G,  Fig.  139,  touch  alter- 
nately A  and  B  with  the  free  end.  If  the  two  deflections 
are  the  same,  and  abnormally  high,  it  indicates  a  ground 
on  both  mains;  if  there  is  no  de- 
flection at  all,  or  if  the  deflections 
are  equal,  but  low,  it  indicates  a 
clear  line,  except  in  so  far  as  leakage 
has  its  influence.  In  order  that 
even  these  indications  may  be 
reliable,  instruments  must  be  se- 
lected to  suit  the  conditions.  As  an  aid  to  this  selec- 
tion the  following  discussion  may  not  be  out  of  place. 

If,  as  in  Fig.  140,  we  suppose  that  we  have  a  telegraph 
line  extending  from  A  to  B,  and  supported  on  ordinary  in- 
sulators having  a  resistance  of  many  millions  of  ohms  each, 
we  might  expect  all  the  current  from  the  battery  at  A  to 
pass  through  the  primary  of  the  relay  at  B,  but  it  is  a 
well-known  fact  that  it  does  not,  hence  the  necessity  of 
the  relay.  The  reason  for  this  is  that  although  the  leak- 
age over  the  surface  of  a  single  insulator  might  be  very 
small,  when  we  have  a  great  many  insulators  in  use 
more  leakage  paths  are  placed  in  multiple,  and  on  long 
lines  so  much  current  leaks  from  positive  to  negative  by 
way  of  the  by-paths  indicated  by  the  dotted  lines,  that 
that  which  reaches  B  is  insufficient  to  operate  the 
sounder  unless  the  relay  is  used,  and  this  leakage  effect 
is  most  marked  in  damp  weather.  Again,  if  we  take  a 
commutator  completed,  and,  by  means  of  one  of  the 
voltmeter  methods  already  described,  measure  the  re- 
sistance of  the  insulation  between  each  bar  and  the 


GROUNDS   ON    THE    LINE. 


443 


shell,  it  will  be  found  to  be  very  high.  Probably  the 
needle  of  the  voltmeter  will  just  wiggle;  if  now  by  means 
of  a  fine  bare  wire  passed  several  times  around  the  com- 
mutator we  connect  all  the  bars  together,  the  insulation 
to  the  shell  will  be  much  lower,  as  indicated  by  the  in- 
creased deflection.  Each  section  of  an  armature  winding 
will  test  high  to  the  core,  but  the  completed  armature 
will  test  much  lower,  and  toothed  armatures  test  lower 
than  smooth  ones  because  more  surface  of  wire  is  exposed 
to  the  core.  Take  two  500  volt  test  lines  and  hold  them 


1 

FIG.  140. 

on  opposite  sides  of  a  sheet  of  asbestos  1/16  inch  thick, 
and  with  no  previous  drying  a  small  deflection  will 
obtain;  next  place  the  sheet  of  asbestos  on  a  smooth 
iron  surface  and  put  a  second  sheet  of  iron  upon  it,  and 
test  between  the  iron  surfaces.  The  deflection  will  be 
found  to  have  greatly  increased,  and  to  a  degree  depend- 
ing upon  the  extent  of  the  surfaces  exposed. 

We  thus  appreciate  the  fact  that  many  high  resistance 
paths  placed  in  multiple  soon  afford  a  path  of  tolerably 
low  resistance,  and  it  is  more  easily  seen  how,  on  exten- 
sive networks  of  wiring  supported  on  insulators,  fed  from 
many  machines,  and  regulated  from  switchboards  con- 
taining many  devices  of  large  surface,  the  insulation  to 
earth  may  be  as  low  as  50  or  100  ohms.  On  the  other 
hand,  on  less  complicated  systems  the  insulation  to  earth 


444  TESTING    OF    DYNAMOS    AND    MOTORS. 

may  run  up  into  thousands  of  ohms.  Between  these 
limits  some  instruments  are  better  adapted  than  others 
for  testing  insulation  during  hours  of  service.  We  know 
that  the  leakage  path  on  any  system  has  a  definite 
resistance,  which  can  be  expressed  in  ohms.  Suppose, 
Fig.  138,  that  the  insulation  resistance  from  A  to  earth 
is  50  ohms,  and  that  that  from  B  to  earth  is  also  50  ohms: 
then  the  total  earth  resistance  from  A  to  B  is  100  ohms, 
and  a  line  voltage  of  100  would  cause  a  leakage  current 
of  i  ampere,  but  the  total  insulation  resistance  of  the 
system  is  that  of  the  two  leakage  paths  in  multiple,  or  25 
ohms,  and  this  is  the  value  we  would  get  were  we  to 
measure  the  system's  insulation  after  working  hours  by 
means  of  a  voltmeter. 

Let  us  suppose  that  we  have  two  mains,  A  and  B,  Fig. 
141,  and  that  one  of  them  is  free  from  leakage  and  per- 
fectly insulated,  but   that  the  other 
has  leakage  paths  whose  aggregate 
resistance  is  56  ohms.     So  long  as 
B  remains  perfectly  insulated  there 
will  be  no  leakage  current,  because 
there   is  no  path  through  which  it 
FIG.  141.  can  reach   B.     Now    suppose  with 

one  terminal  earthed,  we  place  the 

other  terminal  of  a  1,000  ohm  voltmeter  upon  B,  thus 
grounding  B  through  the  resistance  of  the  voltmeter. 
There  is  then  a  complete  circuit,  A,  G',  G,  V,  B,  through 
the  leak  and  the  voltmeter,  and  the  two  are  in  series,  so 
that  the  line  voltage  divides  between  them  proportionately 
to  their  respective  resistances.  Since  the  meter  resistance 
is  20  times  that  of  the  leak,  20  times  as  great  a  drop  will 
take  place  across  it;  /.  ^.,  if  the  potential  difference  be- 


GROUNDS   ON    THE    LINE.  445 

tween  A  and  B  is  105  volts, the  meter  will  show  100  volts, 
the  remaining  5  volts  dropping  through  the  leak,  whose 
resistance  we  will  call  r.  In  other  words,  in  this  particular 
case,  the  drop  through  each  is  proportional  to  its  resist- 
ance. Call  the  meter  resistance  R,  the  line  voltage  F, 
and  the  meter  deflection  d.  Then  in  any  case  the  drop 
across  r  will  be  V  -  d,  and  from  the  above  analysis  we 
have  the  proportion 

d  :    V  -  -   d  ; ;   R   :   r, 
whence 


We  know  or  can  ascertain  R,  we  get  d  in  the  test,  and 
can  measure  V.  The  above  case  is  assumed,  to  simplify 
reasoning,  and  is  not  likely  to  arise  in  practice,  except  on 
isolated  plants.  In  the  every-day  case  where  A  and  B 
are  both  more  or  less  faulty,  the  tests  consists  in  touch- 
ing the  free  end  of  the  voltmeter  alternately  to  one  main 
and  then  the  other,  and  observing  the  two  deflections. 
If  one  meter  terminal  is  fixed,  the  meter  must  either  read 
both  ways  from  a  zero  in  the  centre,  or  must  be  provided 
with  a  reversing  switch,  because  moving  the  free  end 
from  one  main  to  the  other  reverses  its  current.  The 
insulation  resistance  in  this  case  is  not  so  easily  derived 
from  the  value  of  the  deflections,  because  here  the  volt- 
meter not  only  has  a  fault  in  series  with  it,  B  G',  Fig.  142, 
but  has  also  a  fault  in  multiple  with  it,  A  G" ,  The  reader 
will  very  readily  conceive  of  the  two  faults  forming  a 
closed  circuit,  A,  G" ,  G',  B,  between  the  two  mainland 
he  might  further  consider  the  taking  of  the  drop  between 


446  TESTING    OF    DYNAMOS    AND    MOTORS. 

A  and  G  and  between  B  and  G,  to  be  simply  dividing 
into  two  operations  what  might  be  gotten  by  at  once 
taking  the  drop  between  A  and  B.  The  two  faults  do 
form  a  closed  circuit,  and  moreover  the  drop  from  A  to  G 
plus  that  from  G  to  B  does  equal  that  between  A  and  B, 
as  can  be  proven  by  using  an  elec- 
trometer instead  of  a  voltmeter.  An 
electrometer,  being  an  open  circuit 
instrument,  is  of  infinite  resistance, 
and  allows  no  current  flow;  whereas 
the  voltmeter,  being  a  closed  circuit 
instrument,  does  not  read  the  drop  on 
the  fault  alone,  but  reads  the  drop 
on  the  combined  resistance  of  itself  and  the  fault  in 
multiple.  Suppose,  for  instance,  the  voltmeter  resistance 
to  be  60,000  ohms,  that  of  each  fault  to  be  the  same, 
and  the  line  voltage  to  be  500.  Ordinarily,  then, 
there  would  flow  through  the  two  faults  in  series  a 
current  of 


—  ^°_  —  I  _  - 

1200007 


ampere. 


120,000 

As  soon  as  the  voltmeter  is  connected  from  either  main 
to  earth,  it  shunts  one  fault  and  the  combined  multiple 
resistance  of  the  fault  and  meter  is  only  30,000  ohms,  and 
the  meter  registers  the  drop  on  this.  The  total  resistance 
in  the  test  circuit  is  then  60,000  -f-  30,000  =  90,000  ohms, 
and  the  leakage  current  becomes 

=  .0055  ampere, 
90,000 

which  causes  across  the  voltmeter  terminals  a  drop  of 
(/  X  R  =  .0055  X  30,000)  =  165  volts.  Transferring 


(5°°    I 
00007 


GROUNDS    ON    THE    LINE.  447 

the  free  end  of  the  meter  to  the  other  main,  we  get  a  like 
drop,  making  the  sum  of  the  readings  (d ^  -f-  ^2)  330  volts. 
But  we  know  the  potential  difference  between  A  and  B 
to  be  500  volts,  therefore  there  is  a  difference  of  170  volts 
between  the  sum  of  the  successive  readings  taken  above 
and  the  line  voltage,  and  a  voltmeter  would  be  poorly 
adapted  to  measure  the  potential  difference  between  two 
widely  separated  mains,  by  taking  their  drops  to  earth.  • 
An  electrometer  would  serve  this  purpose.  On  the 
other  hand,  an  electrometer  would  not  answer  the  pur- 
pose in  the  insulation  measurement,  because,  without  a 
knowledge  of  the  current  flowing  through  the  two  faults, 
it  would  give  no  indication  of  use  in  determining  the 
absolute  fault  resistance,  and  would  only  indicate  the 
relative  resistances  of  the  two  faults.  When  the  volt- 
meter is  used,  its  application  in  the  test,  as  we  have  seen, 
disturbs  existing  conditions,  and  it  is  this  disturbing 
influence,  together  with  its  causes,  that  enables  us  to 
derive  an  expression  giving  the  insulation  resistance 
of  either  or  both  mains.  From  the  above  course  of 
reasoning  the  conclusion  follows  that  the  lower  the 
resistance  of  the  instrument  the  greater  will  be  its 
modifying  influence,  and  if  the  instrument's  resist- 
ance is  negligibly  low,  it  practically  short  circuits 
the  fault  with  which  it  is  in  multiple,  and  the  leakage 
current  which  flows  is  limited  in  value  only  by  the 
resistance  of  the  meter  and  the  fault  with  which  it  is 
in  series.  Voltmeters  of  negligible  resistance  are  not 
obtainable,  but  if  we  use  an  ammeter,  the  test  is  reduced 
to  the  conditions  of  Fig.  141,  and  nearly  the  same  course 
of  reasoning  follows.  Suppose,  for  example,  that  the 
voltmeter  of  Fig.  141  is  replaced  by  an  ammeter  of  .001 


448  TESTING    OF    DYNAMOS    AND    MOTORS. 

ohm  resistance,  and  suppose  that  with  the  free  end  of  the 
meter  on  B,  a  current  of  i  ampere  flows.  Now  since 
the  resistance  of  the  meter  is  so  low  as  to  short  circuit 
whatever  fault  might  exist  on  B,  this  fault,  unless  itself 
of  very  low  resistance,  would  have  no  appreciable  in- 
fluence upon  the  total  current  flowing,  and  this  current 
would  be 

I-      E 

-r+le' 

where  E  is  the  line  voltage,  r  the  resistance  of  the  fault 
on  A,  and  R  that  of  the  meter.  From  this  expression  we 
get 

,-*_* 

In  dealing  with  a  network  about  whose  resistance  of 
insulation  we  know  nothing,  it  is  well  to  start  by  placing 
in  series  with  the  meter  such  an  extra 
resistance  as  will  limit  the  current  to  the 
meter's  capacity,  if  there  happens  to  be 
a  short  circuit  on  one  main  when  the 
free  end  of  the  meter  is  placed  on  the 

i other.     This  method   is   only  absolutely 

u      G      Q  . 

_,  correct   when    the    meter  resistance  can 

SflG.   143. 

be  neglected  in  comparison  to  that  of  the 
fault  with  which  it  is  in  multiple. 

We  will  now  derive  formulae  applicable,  whatever  may 
be  the  relation  between  the  ammeter  and  the  fault  with 
which  it  is  in  multiple.  In  Fig.  143,  let  A  and  B  be  two 
mains  between  which  a  potential  difference  of  E  volts 
exists.  Let  the  resistance  of  the  ammeter  be  r.  Call 
the  resistance  of  the  faults  on  A,  x,  and  that  on  B,  y. 


GROUNDS    ON    THE    LINE.  449 

Let  d  be  the  ammeter  reading  from  A,  and  d0  that  from 
B.     Then 


is  the  quantity  by  which  d  must  be  multiplied  in  order  to 
express  the  total  leakage  current  value  in  the  first  posi- 
tion of  the  meter,  and 


is  the  multiplier  in  the  second  position  when  we  get  a  de- 
flection d0.      Now  the  resistance  of  r  and  x  in  multiple  is 

i  r  x 


r      x 

and  the  resistance  of  rand  y  in  multiple,  is 
I  ry 


Therefore,    in  the  first  position  of  the  meter  the  total 
test  circuit  resistance  is 


rx 


and  in  the  second  position,  it  is 


450  TESTING    OF    DYNAMOS   AND    MOTORS. 

Since  from  Ohm's  Law 

'-* 

and  since  in  the  first  case  the  expression  for  current  is 

'-^. 

and  in  the  second  case 


we  have, 
x  +  r  _  E          _  E  E  (r  +  x) 


ry  +  xy       rx  +  ry  -+-  xy 


r  +  x 
dividing  both  sides  of  the  equation  by  x  -J-  r,  we  get 

d_  _  E 

x       r  x  -\-  r  y  -\-  xy ' 
Whence, 

_  dry  . 

E  —  d  r  —  d  y 

Also, 


ry 


and  by  the  above  processes,  we  find  that 


d0  r  x 

E  —  dn  r  —  dn  x ' 


GROUNDS   ON    THE    LINE.  451 

Substituting  this  value  of  y  in  equation  (i),  we  get 

dr  -= -j- — 

_  E  —  d0  r  —  d0  x 


E  -  dr- 


E  -  d  r  -  d  x 


E  -  d0r  -  dQx 


dd0  rx 
E  —  dr 


E  -  d0r  -  dn  x 


•**-=— i. 


dd0r    x  _ 
-Ed0x-  Edr+dd0r*' 


Clearing  of  fractions,  we  have, 

E*x  -.Ed0rx  -  Ed0x*  -  Edr  x  +  afe^^^Tz^^i^, 

whence,  leaving  out  canceled  quantities, 

E*  x  -  Ed0rx  -  Ed0x*  -  Edrx  =  o. 
Dividing  through  by  the  common  factor  Ex,  we  have 

E  —  d0r  —  d0x  —  dr  —  Q, 
transposing, 

E  -  d0r  -  dr  =  d0x, 
whence, 

_E-d0r-dr_E  -r(d0+d) 
d0  dQ 

and  this  value  of  x  is  the  insulation  resistance  from  main 
A  to  the  earth.     If  instead  of  substituting  the  value  of  y- 


452  TESTING    OF    DYNAMOS    AND    MOTORS. 

in  the  equation  for  xt  we  had  substituted  the  value  of  x  in 
equation  (2)  for  7,  the  result  would  have  been  the  insula- 
tion resistance  of  B  to  earth. 

Having  the  insulation  resistance  of  each  side  of  the 
line,  the  insulation  of  the  whole  network  is  the  multiple 
resistance  to  earth  of  both  sides. 

x=£-r(^  +  J) (3) 

and 

y  =  -    r-^r-  — w 

Calling  S  the  total  line  insulation,  we  have: 
i  i 


S  = 


1  +  1  i 

^  ~ 


y       E  —  d0r  —  d  r~  E  -  d0r  —    d  r 


E  —  dn  r  -  dr   '     E  —  dn  r  —  d  r       E  —  dnr  —  d 


E-r(d0+d) 

-3C+7- 

A  result  which  can  of  course  be  used  to  obtain  directly 
the  line  insulation  resistance  without  finding  that  of  each 
main.  The  use  of  a  resistance  coil  in  series  with  the 
ammeter  will  not  alter  the  mathematical  accuracy  of  this 
method;  r  then  stands  for  the  meter  resistance  plus 
that  of  the  coil. 

When  using  a  voltmeter  to  make  this  test,  the  analysis 
is  a  little  different,  because  as  we  know  nothing  of  the 


GROUNDS    ON    THE    LINE.  453 

currents  passing  through  either  the  fault  or  the  meter 
we  can  find  no  expression  of  their  relationship;  but  we 
can  find  an  expression  giving  the  distribution  of  the  line 
voltage  in  the  test  circuit:  /.  ^.,  how  much  drops  through 
the  multiple  resistance  of  the  meter  and  one  fault,  and 
how  much  drops  through  the  fault  which  is  in  series  with 
both.  Let  us  use  the  lettering  of  Fig.  143,  but  suppose  the 
ammeter  to  be  replaced  by  a  voltmeter  of  resistance  r. 
The  path  of  leakage  is  A,  x,  G' ',  G'ty,  B,  as  before.  In 
thus  defining  the  path  of  leakage  we  do  not  wish  to  imply 
that  the  leakage  all  takes  place  between  the  points  indi- 
cated on  the  diagram,  for  as  a  matter  of  fact  it  takes 
place  all  along  the  line,  although  a  specific  fault  might 
greatly  localize  it.  Assuming  the  free  end  of  the  volt- 
meter to  be  touched  to  A,  the  multiple  resistance  of  r 
and  x  is 


and  the  voltmeter  indicates  the  drop  across  this  resist- 
ance, the  remainder  taking  place  across  fault  y.  The 
total  resistance  of  the  test  circuit  is  then 


~+~ 


when  the  test  line  is  transferred  to  main  B,  the  test  cir- 
cuit resistance  is 


454  TESTING    OF    DYNAMOS    AND    MOTORS. 

Since  the  potential  falls  from  A  to  B  and  distributes 
itself  in  proportion  to  the  resistance  it  meets,  we  have  in 
either  case  the  fact  that  the  drop  across  the  meter  is  to 
the  drop  across  the  fault  in  series  with  the  meter  as  the 
resistance  between  the  meter  terminals  is  to  the  resist- 
ance of  that  fault,  or,  expressed  as  a  proportion. 


i.i  r  --  x          r  x 


r       x  d  r  x  r  -f-  x  _         r  x 

y       ~^^~      y  y       ~  ry  + 

whence  clearing  of  fractions, 

r  x  (E  —  d)  =  dry  -j-  d  xy, 
r  x  E  —  rxd=  dry  -\-dxy, 
rxE  —  rxd—  dxy  =  dry, 
x  (r  E  —  rd  —  d  y)  =  dr  y; 

X  =  rE-7d-dy 

Transferring  the  meter  to  B  main,  we  have, 
i  i 


T        i_        r  +  y  ry 

~~  i    .. 


d0  r .  '  y  ry  r  +  y  r  y 


E-d0~       x  x  x  rx+xy 

Clearing  of  fractions, 

ry(E-d0)  =  d0(rx  +  xy). 
Multiplying  out, 

ryE  —  ryd0  =  d0r  x  +  d0xy. 

Transposing, 

ryE  —  r yd0  .-  dQxy  =  d0r  x. 


GROUNDS   ON    THE    LINE.  455 

Factoring, 

y  (r  E  —  r  dQ  -  d0  x)  =  d0  r  x. 
Dividing, 

_  d»r  x  ,  * 

y  '"  TE~^r  d0  -d0x' 

Substituting  this  value  in  equation  (6),  we  have, 


r  E  —  r  dQ  —  d0  x 


_  d  d0   rx 

r  E  —  r  d  — 


r  E  —  r  d0  —  d0  x 


-  rEd0x  -  r* 


-  S£9  -  r*  Ed0  -  rEd0x  -  r*  E  d  +  r*d 
Clearing  of  fractions, 

t*  E*x  -  r*Ed0x  -  rEd0x*  -  r*  E  d  x  + 
=^/  yo  P*  uv.      Dividing  by  r  x  E, 

r  E  —  r  d0  —  d0x  —  r  d  —  o, 
or 


If  instead  of  substituting  the  value  of  y  in  equation 
(6),  we  had  substituted  the  value  of  x  in  equation  (7),  the 
result  would  have  been  a  value  of 


456  TESTING    OF    DYNAMOS    AND    MOTORS. 

Having  these  two'values  we  can  as  before  derive  a  for- 
mula for  giving  at  once  the  insulation  of  the  whole  system : 


±    i_i       i 

x        v        rE  —  rdn  —  rd*    rE  —  rd^  —  rd 


d 


rE  —  rd0  —  rd         rE  —  rd0  —  rd       rE  —  rd0  —  rd 

rE  -rda-rd  _r[E-  (4,  +  *)] 
d0  +  d  d,  +  d 

Fig.  144  gives  the  connections  for  a  detector  on  an 
alternating  circuit  system  where  it  is  desirable  to  avoid 
a  permanent  ground.  A  and  B  are  the  two  mains  be- 
tween which  exists  an  alternating  E.  M.  F.,  and  i  and  2 
are  the  primaries  of  two  transformers,  which  are  in  no 
way  connected  to  each  other,  but 
K  i?  B  either  of  which  can  by  means  of 

switch  K  be  put  to  earth  at  G.       L 

and  L  are  lamps  in  series  with  the 

respective     secondaries,    but    have 
no    metallic    connection     with    each 
FIG.  144.  other.    Suppose  a  ground  to  occur  on 

B  at  G ';  upon  throwing  K  over  to  2, 

there  would  be  no  positive  indication,  because  the  result 
would  be  simply  to  ground  both  sides  of  primary  No.  2, 
one  ground  at  G,  the  other  at  G'.  Upon  putting  K  to 
i,  however,  L  will  light  up,  for  we  now  establish  a  path 
— Ay  i,  G,  G',  B — by  means  of  which  current  can  pass 
from  A  to  B,  thus  exciting  primary  i,  which  induces  in 


GROUNDS    ON    THE    LINE.  457 

its  secondary  current  enough  to  light  L.  A  fault  on  A, 
as  at  G",  would  give  no  positive  indication  till  K  touched 
2,  when  path  B,  2,  G,  G",  A,  including  primary  2,  would 
be  established.  The  parts  of  the  detector  can  be  so 
arranged  that  the  lamp  nearest  the  faulty  main  always 
lights. 

The   alternating   system  detector  described   below  is 
due  to  Picou.      Depending  for  action  upon  the  principle 
of  the  condenser,  it  affords  a  safe  means  of  continuous 
indication,    involving    no    metallic    connection    between 
either    main     and    the     ground.      A    and    B    are    the 
mains,  Fig.  145;  M'  and  N'  are  the  inside  coatings  of  two- 
condensers   connected    to  A   and  B   re- 
spectively; M  and    N,   the   outer  coat-        A     ,         ,  B 
ings,  are  connected  together;    from  their       Q-|  h-jH  f-Q 
junction,     and     including     a     telephone 
receiver,    Tt    runs    a    ground    wire,    G. 
Under  ordinary  circumstances,  the  'phone 
emits   little   if  any  sound,  but  upon  the 
occurrence     of     a     ground    on     either  FIG.  145. 

main,  a  distinct  buzzing  is  heard,  and 
this  can  be  very  much  intensified  by  the  use  of  proper 
devices.  The  detector  can  be  made  to  indicate  the 
faulty  main  by  arranging  switches  to  successively 
remove  the  condensers  from  circuit.  Suppose  a 
ground  to  occur  at  G'  on  A;  if  condenser  M'  M  is 
removed  from  circuit,  the  'phone  will  continue  to  buzz, 
as  it  is  still  in  circuit  with  N'  JV,  one  of  whose  coatings 
goes  directly  to  main  B,  the  other  through  the  ground 
to  A.  If  M'  M  is  replaced  and  N'  N  removed,  T  will 
not  buzz,  because  its  terminals  are  in  circuit  with  a  con- 
denser both  of  whose  coatings  are  to  earth.  The  fault 


458 


TESTING    OF    DYNAMOS    AND    MOTORS. 


is  always  found  in  the  main  attached  to  the  condenser 
whose  removal  fails  to  stop  the  buzzing  of  the  'phone. 

On  arc  light  and  other  series  systems,  the  two  mains 
are  seldom^  together  except  where  they  leave  and  enter 
the  power  house.  A  detector  arrangement  for  such 
a  circuit  is  shown  in  Fig.  146,  where  A  is  the  constant 

current  dynamo,  Ll  Z2,  etc.,  the 
lamp  load.  As  it  is  not  safe  to 
permanently  earth  a  high  po- 
tential system;  a  switch  is  used 
to  temporarily  make  this  con- 
nection. It  is  customary  to  use 
a  water  or  gas  pipe  as  the 
ground,  and  the  ground  wire 
should  include  a  high  resistance 
voltmeter,  by  means  of  which 
can  be  told  the  number  of 

lamps  included  between  the  fault  and  the  detector.  If 
each  lamp  is  adjusted  to  regulate  for  a  certain  current 
and  voltage,  say  10  amperes  and  50  volts,  and  with  the 
current  right  the  detector  voltmeter  should  register  300 
volts,  it  would  indicate  the  fault  to  be  just  beyond  the 
sixth  lamp  from  the  detector.  If  a  dead  ground  exists 
on  an  arc  lamp  circuit,  the  effect  of  a  second  ground  is 
to  cut  out  all  lamps  included  between  the  two  grounds. 
In  the  present  case  the  high  resistance  of  the  voltmeter 
circuit  obviates  this,  with  the  result  that  the  meter  reads 
the  drop  on  those  lamps  included  between  the  two 
grounds.  Every  arc  machine  is  supposed  to  maintain 
constant  current,  whatever  the  load,  so  in  this  case  the 
voltmeter  reading  depends  directly  upon  the  number  of 
lamps  included  between  its  terminals.  As  the  meter 


FIG.  146. 


GROUNDS   ON    THE    LINE. 


459 


may  .be  called  upon  to  register  very  high  voltage,  it 
either  must  be  very  high  reading  in  itself  or  must  be 
provided  with  a  multiplier.  On  arc  circuits  contain- 
ing a  great  many  lamps,  it  is  customary  to  arrange 
them  in  sections,  so  that  in  case  of  grounds  serious 
enough  to  cut  out  lamps  the  faulty  sections  can  be 
readily  located.  A  single  external  ground  will  not 
impair  an  arc  service  unless  the  machine  itself  be 
grounded. 

Fig.  147  gives  an  ingenious  form  of  ground  detector 
recently  put  on  the  market  by  the  Stanley  Electric 
Manufacturing  Co.  of  Pittslield, 
Mass.,  and  is  unique  in  that  it 
not  only  absorbs  no  energy, 
but  requires  no  metallic  connec- 
tion whatever  between  earth  and 
line.  It  is  a  long-known  fact 
that  attraction  exists  between 
objects  at  different  electrical  po- 
tentials, that  if  two  metallic 
plates  hung  from  two  fine  wires 
attached  respectively  to  the  posi- 
tive and  negative  poles  of  any  source  of  E.  M.  F. 
be  free  to  move,  they  will  move  toward  each  other, 
and  it  is  upon  this  principle  that  the  electrometer 
is  built.  The  detector  in  question  is  a  development  of 
this  idea,  and  consists  of  four  fixed  metallic  vanes,  and 
a  movable  vane  made  of  aluminum  to  secure  lightness. 
The  movable  vane  carries  a  pointer  which  ordinarily 
rests  at  zero.  Two  of  the  fixed  vanes  go  to  one  side  of 
the  line  to  be  indicated;  the  remaining  two  are  con- 
nected to  the  opposite  side,  and  the  movable  vane  goes 


460  TESTING    OF    DYNAMOS    AND    MOTORS. 

to  earth.  The  movable  vane  is  oppositely  attracted  by 
each  pair  of  fixed  vanes,  and  ordinarily  is  at  zero  on  the 
scale.  If  a  ground  occurs,  however,  the  effect  is  to 
establish  connection  between  the  movable  vane  and  one 
pair  of  the  fixed  ones,  and  the  pointer  moves  over 
toward  the  faulty  main. 

The  existence  of  a  ground  being  assured,  the  method  of 
locating  it  depends  upon  the  facilities  at  hand  for  test- 
ing, upon  the  complexity  of  the  wiring,  and  upon  the 
gravity  of  the  fault.  If  the  fault  occur  on  one  of  such 
a  network  of  wires  as  is  found  on  city  incandescent  sys- 
tems, any  or  all  of  the  following  methods  can  be  used  to 
locate  it:  i.  Send  an  expert  lineman  out  on  a  tour  of 
inspection.  This  may  sound  very  commonplace,  but  it 
has  succeeded  in  instances  while  finer  methods  were  in 
course  of  preparation.  2.  The  different  branches  of  the 
system  can  be  isolated  and  the  faulty  one  located  by 
means  of  a  bridge,  a  magneto,  or  a  voltmeter  with  some 
insulated  source  of  E.  M.  F.  3.  The  fault  can  be 
burned  out. 

A  tour  of  inspection  often  reveals  some  defect  in  insu- 
lation— a  broken  insulator,  a  naked  feeder  making  con- 
tact with  a  tin  roof  or  iron  bridge  or  wet  trunk  of  a  tree. 
If  the  system  be  underground  inspection  cannot  be  very 
extensive,  and  one  of  the  other  methods  must  be  used. 
If  method  No.  2  is  resorted  to,  the  engineer,  with  a 
map  of  the  system  before  him,  decides  which  wires  can 
be  cut  most  conveniently.  Suppose  the  system  is  divided 
into  halves;  the  test  will  show  in  which  half  the 
fault  lies.  The  faulty  half  can  be  again  halved,  and  the 
position  of  the  fault  found  to  a  further  approximation:  by 
further  subdivision  the  limits  are  further  narrowed  till 


GROUNDS    ON    THE    LINE.  461 

they  become  small  enough  to  make  inspection  practicable. 
On  overhead  work  the  magneto  is  generally  satisfactory, 
but  on  very  extensive  systems  and  on  underground 
work  it  is  not  always  reliable,  because  the  magneto's 
current  is  alternating,  and  in  successively  charging  and 
discharging  a  system  of  great  capacity,  sufficient  current 
would  flow  to  ring  the  bell,  thus  indicating  a  ground 
though  none  existed.  An  ordinary  magneto  turned  by 
hand  will  give  an  indicated  voltage  of  from  50  to  100 
volts,  but  this  can  be  greatly  increased  by  belting  the 
armature  to  a  hand  wheel  of  larger  diameter,  or,  where 
practicable,  by  belting  to  a  shop  shafting.  Connections 
for  ringing  out  a  fault  are  given  in  Fig.  148,  where  A  B 
is  the  isolated  section  of  line;  a  ground  exists  at  G '. 
M  is  the  magneto,  one  of  whose  terminals  goes  to  a  gas 
or  water  pipe  and  the  other  to  A  B.  Upon  turning  the 
handle  current  flows  through  circuit  G,  G',  B,  A^  M,  G. 
Without  any  ground  at  G '  the  circuit 
would  be  incomplete,  and  the  silence 
of  the  bell  would  indicate  good  insu- 
lation, assuming  the  bell  to  be  in  good 
order.  To  begin  with,  the  leads  are 
held  apart  and  the  armature  turned  to 
insure  that  no  cross  exists  in  the  leads 
or  inside  connections.  The  test  is  re- 
peated with  the  leads  held  together;  the 
bell  should  of  course  ring.  The  magneto  can  be  re- 
placed by  a  bridge  or  voltmeter. 

On  underground  work  more  delicate  instruments  are 
generally  used.  Except  at  junction  boxes,  the  cables  are 
open  to  inspection  only  by  digging,  and  it  is  a  matter  of 
much  importance  to  locate  the  exact  seat  of  the  trouble 


462  TESTING    OF    DYNAMOS    AND    MOTORS. 

before  opening  the  street.  In  locating  cable  grounds  the 
test  is  much  simplified  by  isolating  as  far  as  possible  the 
cable  under  test.  If  in  cable  A  D  B,  of  Fig.  149,  a  fault 

were     located     apparently    at    D,    it 

would  be  impossible,  from  the  nature 
.    of  the  test,  to  decide  if  the  fault  lay 

at  D  or  at  some  point  on  the  branch 
~B      cable  E  D  C;  if  disconnecting  E  D  C 

removes  the  fault,    then   E  D  C  is  the 
(;  seat  of  trouble.     The  test  consists  in 

F  sending  a  steady  current  through  A  D 

B  by  means  of  a  dynamo,  storage 
battery,  or  other  steady  source  of  E.  M.  F.,  and  with  a 
sensitive  galvanometer  taking  successively  the  drop  of 
potential  between  A  and  the  earth  and  B  and  the  earth. 
The  metal  sheathing  of  the  cable  makes  a  good  earth. 
The  ratio  of  the  two  deflections  is  the  ratio  of  the  num- 
ber of  feet  included  between  the  fault  and  the  respec- 
tive ends  of  the  piece  of  cable  under  test,  so  that 
knowing  the  total  length  of  the  piece  enables  one  to 
exactly  locate  the  fault. 

In  Fig.  150  let  A  B  be  a  section  of  cable  150  feet  long, 
and  suppose  a  ground  to  exist  at  G.  Y  is  the  battery 
and  g  the  galvanometer.  At  G,  then,  the  conductor 
comes  in  contact  with  the  ground,  and  at  that  point 
assumes  the  earth's  potential.  G,  G',  G",  being  all 
grounds,  are  practically  at  the  same  potential,  so  that 
placing  the  galvanometer  terminals  on  A  and  G',  or  on 
B  and  G",  is  the  same  as  placing  one  on  G  and  the 
other  successively  on  A  and  B.  Throughout  the 
circuit — Y,  A,  Z>,  B,  F— there  is  a  drop  of  potential, 
and  part  of  this  drop  takes  place  from  B  to  G 


GROUNDS   ON    THE    LINE.  463 

and  part  from  G  to  A.  Suppose  the  galvanometer 
deflects  60  divisions  due  to  the  drop  between  A  and 
G,  and  40  divisions  due  to  the  drop  between  G  and  B. 
To  locate  the  fault  we  must  divide  the  total  length  (150 


Y 

1  il  I 

1  H  1 
A                                    D 

I 

k               I 

5 

G'                                  Q 
FIG.  150. 

Q" 

feet)  of  the  cable  into  two  parts  that  shall  be  to  each 
other  as  60  is  to  40,  or  as  3  :  2.  To  get  this  division 
arithmetically,  suppose  the  cable  to  be  divided  into  unit 
lengths,  each  unit  length  containing  such  a  number  of 
feet  that  5  =  (3  +  2)  units  shall  equal  the  total  length 
of  the  cable,  and  in  this  case  =  150  feet.  Each  unit 
length  then  contains  30  feet.  One  piece  of  the  cable 
must  contain  3  of  these  units  =  90  feet,  and  the  other 
part  2  units  =  60  feet;  /.  e.,  the  fault  is  90  feet  from  A 
and  60  feet  from  B. 

Another  method  of  locating  a  fault  to  within  a  foot  or 
so  depends  upon  the  bridge  principle.  While  no  actual 
bridge  is  employed,  the  connections  and  computations 
are  those  of  the  bridge.  The  test  requires  an  auxiliary 
conductor  to  serve  as  a  return. 

Fig.  151  gives  the  actual  connections  for  the  test,  and 
Fig.  152  a  scheme  to  more  readily  identify  the  bridge 
principle.  In  Fig.  151  A  N  S  S'  is  the  return  cable,  and 
CJ/S'the  faulty  one.  Completing  the  circuit  at  the 
station  end  is  a  battery,  B,  including  in  circuit  a  switch, 


464 


TESTING    OF    DYNAMOS    AND    MOTORS. 


K.  In  multiple  with  cables  A  N  S  S'  M  C  is  a  piece  of 
cable,  A  P  C,  of  known  length  and  uniform  cross-section, 
and  corresponding  to  the  proportion  arms  of  an  ordi- 


M  _Js' 


a  "6' 

FIG.  151. 


nary  bridge.  Current  leaving  battery,  B,  has  choice  of 
two  paths,  A  N  S'  C  and  A  P  C,  between  which  it  divides 
in  the  inverse  ratio  of  their  resistances.  M  is  the  fault, 
and  it  is  required  to  find  its  position.  P  G  is  a  wire 
making  earth,  and  including  galvanometer,  g.  The  un- 


FIG.  152. 

grounded  end  of  g  has  a  contact  free  to  slide  along  A  P  C. 
P  is  moved  along  until  a  position  is  found,  such  that 
when  the  battery  and  galvanometer  circuits  are  closed  no 
movement  of  the  needle  is  observed;  under  this  condi- 
tion balance  is  established  and 


GROUNDS    ON    THE    LINE.  465 

A  M  :    CM  ::  AP   :   PC, 

whence  it  follows  from  the  laws  of  proportion  that 
AM  :   A  M  +  CM  ::  A  P   :   A  P  +  P  C. 
But  ,4  M+  CM  -  A  NS  C*n&  AP  +  PC=APC; 
therefore, 

AM  :  A  N  S'  C  :\  A  P   :   A  P  C 
and 


Knowing  the  value  of 

A  P 


A 

it  is  necessary  to  know  the  length,  A  N  S'  C,  and  their 
product  will  be  A  M,  the  distance  in  feet  from  A  to  M. 
Should  A  N  S'  C  contain  two  sizes  of  wire,  its  total 
length  corresponding  to  a  certain  resistance  must  be 
expressed  in  terms  of  one  size  wire  selected  as  the 
standard. 

The  following  method  employs  an  actual  bridge.  Fig. 
153  gives  the  connections.  It  will  be  noticed  that  the 
position  of  the  battery  is  such  that  the  fault  resistance 
has  no  influence  upon  the  accuracy  of  the  test.  A  B  and 
B  C  are,  as  usual,  the  proportion  arms  of  the  bridge,  but 
are,  together,  used  as  a  single  arm,  while  R  is  the  variable 
arm.  Calling  x  the  distance  from  C  to  the  fault,  the 
proportion  under  the  condition  of  balance  is 


x  = 


where  L  is  the  length  of  the  line,  C  x  M,  under  test. 


466 


TESTING    OF    DYNAMOS    AND    MOTORS. 


A  practice  often  resorted  to  in  central  station  work  is 
that  of  burning  out  a  cross  or  ground.  It  is  effective  in 
most  cases,  but  sometimes  the  fault  has  such  great 
current  carrying  capacity  that  the  station  cannot  supply 
enough  current  to  burn  it  out.  To  burn  out  easily  the 


•V                    r    "                   ^_ 

>QC 

^  —  FAULT 
\  \ 

J<                                                    /  s 

R                  /  I 

ft 

\ 

A 
-\ 

/ 

FIG.  153. 

cross  must  offer  some  resistance,  so  that  the  heat  gen- 
erated by  the  passage  of  current  will  be  sufficient  to  fuse 
the  contact.  In  order  for  a  ground  to  give  immediate 
trouble  on  a  metallic  circuit,  a  second  one  must  occur,  so 
that  to  burn  out  a  single  ground  it  is  necessary  to  estab- 
lish artificially  a  second  one.  When  the  detector  indi- 
cates a  ground  which  requires  immediate  removal,  the 
detector  ground  is  replaced  by  one  consisting  of  a  stout 
cable  including  a  heavy  fuse.  The  full  station  over- 
load, if  necessary,  is  then  run  through  the  two  grounds 
until  either  the  fault  or  station  fuse,  or  fuse  nearest  the 
fault,  gives  way.  The  best  way  to  accomplish  this  is  as 
follows:  Use  one  dynamo  to  separately  excite  all  the 
others,  for  if  the  fault  is  a  short  circuit,  shunt  machines 
in  circuit  with  it  will  not  support  a  field.  A  light  field  is 
put  on  all  the  dynamos,  and  their  armatures  must  all  be 


GROUNDS   ON    THE    LINE.  467 

in  multiple.  To  render  the  station  inactive  it  is  only 
necessary  to  pull  the  exciter's  switch.  Next,  put  in  a  fuse 
that  will  carry  40  or  50  per  cent,  overload,  and  load  the 
station  till  this  fuse,  or  something  else,  gives  way.  When 
starting,  be  certain  that  all  machines  are  of  the  same 
polarity,  close  the  exciter-switch,  and  observe  the  indi- 
vidual ammeters  to  see  that  no  machine  is  carrying  more 
than  its  share  of  the  load.  If  there  happen  to  be  any, 
their  loads  can  be  regulated  by  means  of  their  rheostats. 
Should  the  fault  give  way,  the  exciter  switch  must  be 
opened  to  avoid  maintaining  any  arc  which  the  burning 
out  of  the  fault  might  establish.  Very  often  the  fault 
does  not  give  way,  but  the  fuse  immediately  in  circuit 
with  it  does,  and  patrons  deprived  of  service  hasten  to 
report  the  fact. 

The  advantage  of  separate  excitation  is  that  the  load 
can  be  worked  on  under  good  control,  and  need  not  exceed 
the  value  necessary  to  remove  the  fault. 


CHAPTER  XIV. 

MOTOR    TESTING. 

ANY  machine  that  can  be  used  as  a  generator  can  be 
used  as  a  motor,  not  excepting  even  arc  machines,  whose 
qualifications,  however,  are  very  unfavorable. 

It  is  a  matter  of  history  that  the  discovery  of  the 
motor  preceded  that  of  the  generator,  and  that  the  prin- 
ciple of  reversibility  was  first  applied  to  transforming  a 
motor  into  a  generator. 

Motors  are  classified  in  the  same  manner  as  are  dy- 
namos; according  as  they  are  series-wound,  shunt-wound, 
or  compound-wound.  They  may  be  unipolar,  bipolar, 
or  multipolar,  according  as  the  number  of  well-defined 
poles  is  one,  two,  or  more.  The  so-called  unipolar 
machine  is  really  bipolar,  one  pole  being  more  prominent 
than  the  other.  The  number  of  poles  dictates  the  num- 
ber of  neutral  points,  and  hence  the  number  of  brushes, 
except  in  special  cases,  where  the  armature  is  cross-con- 
nected. Cross-connecting  is  used  as  a  means  of  reducing 
the  number  of  brushes.  On  a  four-pole  machine  there 
are,  ordinarily,  four  brushes  (one  at  each  neutral  point), 
alternating  in  polarity,  so  that  opposite  brushes  are  of 
the  same  polarity,  and  can  be  connected;  but  adjacent 
brushes,  being  of  opposite  polarity,  must  be  well  insu- 
lated from  each  other.  The  result  of  connecting  like 
brushes  is  to  place  the  four  quarters  of  the  armature 

468 


MOTOR     TESTING.  469 

winding  in  multiple,  thereby  rendering  the  armature's 
current  capacity  four  times  that  of  the  component  wires. 

Now,  since  opposite  brushes  are  at  the  same  potential, 
the  respective  armature  wires  under  these  brushes  are  at 
the  same  potential,  and,  in  fact,  diametrically  opposite 
wires  all  around  the  armature  are  at. the  same  potential, 
and  may  be  profitably  connected,  thereby  doing  away 
with  one  pair  of  brushes,  providing  the  remaining  pair 
can  carry  the  total  current  alone.  This  device,  in  sub- 
stance, constitutes  cross-connecting,  and  consists  in 
bringing  down  to  the  same  commutator  bar  the  leads 
from  all  conductors  which  are  at  the  same  potential. 
This  reduces  the  number  of  bars  required.  Where  it  is 
not  convenient  to  cross-connect  the  armature,  opposite 
commutator  bars  are  connected;  this  serves  the  same 
ends,  so  far  as  the  brushes  are  concerned,  but  does  not 
reduce  the  number  of  commutator  bars  required.  As  the 
number  of  poles  on  a  machine  increases,  complications 
arise  which  lead  to  the  using  of  the  full  number  of 
brushes,  instead  of  cross-connecting.  On  motors,  and 
especially  on  street-railway  motors,  the  practice  of  cross- 
connecting  is  more  generally  adopted,  on  account  of  the 
limitations  of  space  and  accessibility.  On  inclosed  mo- 
tors there  are  but  two  brushes,  and  these  are  easily 
inspected  from  above. 

A  multipolar  armature  of  the  ordinary  type,  if  not 
cross-connected,  must  have  as  many  brushes  as  there  are 
poles.  If  a  smaller  number  be  used  the  armature's  cur- 
rent capacity  is  reduced;  it  becomes  electrically  unbal- 
anced and  heats.  Figs.  154  and  155  will  serve  to  make 
this  more  clear. 

In  Fig.  154,  A  is  a  motor  armature  running  in  a  four- 


470 


TESTING    OF    DYNAMOS    AND    MOTORS. 


pole  field,  and  has  its  opposite  brushes  connected.  The 
arrow-heads  indicate  the  direction  of  the  current  from 
brushes  -j-  b  -\-b  to  brushes  —  b  —  b.  The  current  enter- 
ing by  the  positive  brushes  traverses  one-fourth  of  the 


FIG.  154. 

armature  and  then  leaves  by  the  negative  brushes.     The 
four  quadrants  are  therefore  in  multiple. 

Let  us  assume  that  the  current  passing  under  the  posi- 
tive pole,  Plt  produces  rotation  in  a  clockwise  direction; 
then  that  under  the  negative  poles  P9,  Pt,  will  concur  in 
this  effort,  for  although  the  current  is  flowing  in  the 
opposite  direction,  the  fields  also  are  reversed,  and  their 
combined  effect  is  the  same.  The  current  under  P9 
concurs  with  that  under  Plt  and  thus  all  four  quadrants 
impel  the  armature  in  the  same  direction.  Fig.  154  is 
the  usual  connection  for  a  four-pole  motor.  Fig.  155 


MOTOR     TESTING.  471 

gives  the  same  machine  with  one  pair  of  brushes  re- 
moved. In  this  case  the  several  poles  remain  the  same 
in  polarity  as  before,  but  the  current  under  pole  P%  is 
reversed,  so  that  now  one-quarter  of  the  armature  is  in 
multiple  with  three-quarters,  thus  resulting  in  an  une- 


FIG.  155. 

qual  distribution  of  current  and  in  wasteful  heating. 
The  arrow-heads  of  Fig.  155  show  the  direction  of  the 
current  under  these  circumstances. 

A  motor  differs  from  a  dynamo  in  that  it  has  no  criti- 
cal speed  of  excitation,  its  fields  depending  either  upon 
separate  excitation  or  upon  the  impressed  E.  M.  F. ,  and 
not  upon  any  E.  M.  F.  generated  within  the  armature. 

Series  motors  are  peculiar  in  that  they  can  have  a  field 
only  when  current  flows  through  the  armature,  and  any 
increase  of  armature  current  results  in  additional  field 


472 


TESTING    OF    DYNAMOS    AND    MOTORS. 


••strength  and  an  increased  C.  E.  M.  F.,  thus  providing  a 
factor  of  safety  against  prolonged  short  circuits.  The 
fact  that  armature  and  fields  are  in  series  removes  the 
necessity  of  a  high-resistance  starting  box,  and  on  some 
series  motors  of  early  date  the  fields  are  wound  in  sec- 
tions, which,  when  in  series,  contain  resistance  enough 
to  dispense  with  an  extra  starting  coil.  For  obtaining 
the  several  speeds,  the  sections  are  thrown  into  several 


-f 


FIG.  156. 

combinations  and  are  finally  in  multiple,  the  combination 
of  least  resistance. 

With  shunt  motors,  the  field,  being  in  shunt  with  the 
armature,  is  of  high  resistance,  has  great  self-induction, 
and  hence  takes  longer  to  attain  its  full  value.  In 
starting  up  a  shunt  motor  a  starting  box  is  absolutely 
necessary  to  avoid  a  short  circuit,  and  this  box  must  be 
put  in  series  with  the  armature,  as  shown  in  Fig.  156, 
which  gives  the  usual  method  of  connecting  in  a  starting 
box. 

A  further  precaution  is  to  connect  the  fields  orf  a  shunt 
motor,  or  the  shunt  fields  of  a  compound-wound  motor 
above  the  switch,  so  that  the  motor  may  be  given  a  field 
before  the  armature  circuit  is  closed.  When  the  motor 
is  idle,  both  the  box  and  the  switch  K,  Fig.  157,  are 


MOTOR     TESTING. 


473 


FIG.  157. 


open.  To  start  the  motor  its  field  switch  (not  shown  in 
figure)  is  first  closed.  Then  the  starting  box  is  slowly 
advanced  to  its  final  position.  A" closed,  and  the  starting- 
box  circuit  once  more  opened, 
so  that  simply  by  opening  K 
the  motor  can  be  shut  down. 
The  proper  selection  of  a 
starting  box  depends  upon  the 
object  in  view,  (i)  It  maybe 
desired  simply  that  the  initial 
current  flow  shall  not  exceed 
the  motor's  capacity;  or,  (2) 
that  the  start  shall  be  so 
gradual  as  to  avoid  jarring  the 
mechanism  operated  by  the 
motor.  For  instance,  on  a 
car,  the  first  flow  of  current 

may  be  considerably  below  the  motor's  rated  capacity, 
and  yet  seriously  incommode  passengers.  To  design 
a  resistance  to  meet  the  demands  of  such  variable 
conditions  as  are  encountered  in  street-railway  prac- 
tice, the  engineer  must  effect  a  compromise  between 
theory  and  practice.  Theory  would  have  a  car  start 
smoothly  under  all  conditions  imposed  by  variations 
in  car  weight  and  grade  climbing,  and  \vould  provide  a 
ready  means  to  this  eirl  Practice,  at  least  as  voiced  in 
the  demands  of  most  street-railway  men,  would  have  a 
car  start  on  the  first  notch  of  the  controller  under  all 
conditions.  This  means  that  a  resistance  designed  to 
start  smoothly  a  loaded  car  on  a  grade  will  violently  jerk 
an  empty  car  on  a  level.  In  fact,  starting  coils  are 
designed  to-day  to  give  an  initial  current  flow  equal  to 


474  TESTING    OF    DYNAMOS    AND    MOTORS. 

the  maximum  output  of  one  motor,  or  the  normal  output 
of  both. 

If  the  carrying  capacity  alone  of  the  motor  is  to  be 
considered, the  problem  resolves  itself  to  a  simple  applica- 
tion of  Ohm's  law,  but  a  consideration  of  smoothness  in 
starting  requires  experimental  data.  Knowing  the  cur- 
rent capacity  of  the  motor  to  be  started,  the  resistance 
that  will  just  permit  this  current  to  flow  can  be  gotten 
from  the  equation, 


where  E  is  the  impressed  voltage,  /  the  allowable  cur- 
rent, and  R  the  resistance  value  sought.  The  armature 
resistance  is  usually  so  low  as  to  be  negligible,  and  need 
not  enter  the  equation.  An  important  consideration  is 
to  be  sure  that  the  wire  making  up  the  resistance  has 
enough  cross-section  to  carry  the  current  called  for. 
The  coils  remaining  longest  in  circuit  are  generally  made 
of  heavier  wire  than  those  that  are  cut  out  first.  Table 
IV.  of  the  Appendix  gives  the  current  capacity  of  tinned 
iron  wire  under  different  conditions. 

It  must  be  borne  in  mind  that  a  box  designed  for  one 
particular  voltage  cannot  be  used  safely  on  lines  of  much 
higher  voltage,  nor  will  it  give  the  same  nicety  of  control 
when  used  on  a  line  of  lower  voltage;  because,  in  the  first 
case,  the  excess  of  voltage  is  liable  to  heat  the  box  or 
overload  the  motor,  and,  in  the  second  case,  the  resist- 
ance is  likely  to  be  too  high  to  admit  of  a  ready 
start.  A  box  intended  for  a  motor  of  large  output  can- 
not be  used  with  impunity  on  a  small  motor,  because 


MOTOR     TESTING.  475 

the  starting  current  of  the  larger  motor  would  be 
too  much  for  the  smaller  one;  on  the  other  hand,  a 
box  designed  for  a  small  motor  would  not  admit 
sufficient  current  to  start  a  large  one  without  advanc- 
ing the  handle  to  a  position  dangerous  to  hold  for  any 
length  of  time. 

Starting  boxes  are  preferably  placed  in  multiple  with 
the  switch,  so  that  when  the  resistance  is  all  out  the 
box  maybe  short  circuited  by  closing  the  switch,  and 
the  box  bar  returned  to  the  "off"  position.  Should 
the  box  be  left  in,  both  it  and  the  switch  must  be  opened 
when  it  is  desired  to  stop  the  motor. 

On  shunt  machines,  the  box  is  used  only  in  start- 
ing, but  on  series  machines,  which  race  when  the  load 
is  removed,  it  is  used  to  control  the  speed.  Occasions 
sometimes  arise  for  using  a  street  car  motor  to  run  the 
repair  shop  shafting.  In  such  a  case  the  instability  of 
speed  of  a  series  motor  is  a  serious  drawback  and  it  can 
only  be  overcome  by  replacing  the  series  coils  by  an 
equivalent  shunt  winding.  Suppose  that  we  have  such 
a.  motor  and  that  we  wish  to  change  its  winding.  Let 
the  series  coils,  two  in  number,  consist  of  100 
turns  of  No.  4  B.  &  S.  wire.  Suppose  each  spool 
measures  .175  ohm.  The  total  resistance  then  is  .35 
ohm,  corresponding  to  a  length  of  1,400  feet.  If  full 
load  current  be  50  amperes,  full  load  drop  (I  J?)  is  .35  X 
50  =  17.5  volts,  and  the  watts  lost  in  heat  (e  7)  is  17.5  X 
50  =  875  watts.  With  the  fields  hot  this  loss  would 
increase  to  1,000  watts.  If  the  line  voltage  be  500,  the 
total  intake  of  the  motor  is  500  X  50  —  25,000  watts, 
and  the  field  loss  hot  =  1,000  -^  25,000  =  4  <f>.  Fifty 
amperes  passing  around  200  turns  of  wire  give  50  x  200 


476  TESTING    OF    DYNAMOS    AND    MOTORS. 

=  10,000  ampere-turns  as  the  field  coils'  magnetizing 
force  at  full  load. 

The  shunt  winding  must  provide  this  magnetizing 
force  and  must  not  waste  more  than  4  %  of  the  full 
load  output  in  doing  so.  To  restrict  the  loss  to 
1,000  watts,  the  field  current  at  500  volts  must  not 
exceed  1,000  -^  500  =  2  amperes.  A  current  of  2 
amperes  at  500  volts  calls  for  a  resistance  of  500 
-i-  2  =  250  ohms,  or  125  ohms  per  spool.  The  re- 
quired number  of  ampere-turns  is  10,000,  and  since  the 
amperes  are  2  the  number  of  turns  is  10,000  -=-  2  =  5,000 
or  2,500  per  spool.  Since  the  ampere-turns  are  the 
same,  the  weight  of  copper  in  the  two  cases  will  be 
about  the  same  and  will  approximate  180  pounds,  or  90 
pounds  per  spool.  Now,  to  figure  the  winding  space: 
the  sectional  area  of  a  No.  4  wire  is  .0328  square  inch, 
and  that  of  200  turns  is  200  x  .0328  =  6.56  square 
inches.  Consulting  a  wire  table  we  see  that  5,000  turns 
of  No.  18  wire  gives  a  cross-section  of  (5,000  x  .0013) 
6.5  square  inches,  and  that  a  length  sufficient  to  measure 
250  ohms  (39,000  feet)  weighs  195  pounds.  This  winding 
then  about  fulfills  the  conditions.  As  a  matter  of  fact,  a 
shunt  winding  requires  more  space  than  a  series  winding 
of  the  same  power,  because  not  only  is  the  radiation 
poorer  but  a  larger  per  cent,  of  the  winding  space  is 
occupied  by  insulation. 

The  series  motor's  instability  under  variable  load  can 
be  explained  as  follows:  we  have  learned  that  on  self- 
exciting  dynamos  a  certain  speed  must  be  attained  be- 
fore the  armature  will  build  -up  a  field,  and  it  follows 
that  its  voltage  will  be  proportional  not  to  the  actual 
speed,  but  to  the  actual  speed  less  the  critical  speed. 


MOTOR     TESTING.  477 

Thus  the  voltage  at  two  different  speeds  have  the  follow- 


ing relation  to  each  other: 


where  No  is  the  critical  speed,  and  N^  N^  the  speeds 
corresponding  to  voltages  F,,  Ka  respectively.  This 
proportion  is  true  for  a  considerable  range  of  speed 
variation,  but  is  departed  from  as  the  load  increases 
because  of  the  influence  of  armature  reaction,  and  it  is 
only  by  maintaining  the  armature  current  constant  that 
this  disturbing  effect  can  be  eliminated.  On  separately 
excited  dynamos,  which  have  no  critical  speed,  the 
voltage  is  strictly  proportional  to  the  speed  when  run- 
ning free,  and  when  under  load,  any  change  in  speed 
changes  the  voltage  proportionately  provided  the  arma- 
ture current  is  kept  constant.  If  the  machine  is  run  as 
a  motor,  what  was  true  for  the  dynamo's  voltage  is  true 
for  the  motor's  speed  if  its  field  is  kept  constant.  The 
speed  will  be  proportional  to  the  voltage  and  theoretic- 
ally become  zero  when  the  voltage  does  so,  but  prac- 
tically, a  little  before  this,  on  account  of  internal 
resistance,  friction,  etc.  The  speed  is,  then,  propor- 
tional to  the  impressed  voltage  less  that  voltage  below 
which  the  armature  stops  turning  or  below  which  the 
armature  refuses  to  turn.  Here 

JV::JV,  ::  Vt  -  V,:V,-  V,, 

where  F0  is  the  critical  voltage.  A  separately  excited 
motor  does  not  race  unless  the  voltage  is  abnormally 
high  or  the  field  very  weak. 


TESTING    OF    DYNAMOS    AND    MOTORS. 

The  speed  which  any  motor  attains  is  such  that  the 
following  equation  always  holds  true: 

C.  E.  M.  F.  +  /  ^ft  =  Impressed  E.  M.  F. 

Here  I  /?a  is  the  armature  drop.  A  change  in  any  of 
these  terms  will  alter  the  speed;  thus  an  increased  field 
raises  the  C.  E.  M.  F.,  and  the  speed  falls  until  the 
above  balance  is  restored.  The  effect  of  putting  on 
a  load  is  to  decrease  the  speed,  lower  the  C.  E.  M.  F., 
and  let  in  a  larger  current  and  increase  /  R^  until  the 
balance  again  obtains.  Upon  removing  a  load  the 
retarding  force  is  removed,  and  the  speed  leaps  upward, 
-striking  a  final  value  dependent  upon  the  field  strength. 
This  is  why  a  separately  excited  motor  does  not  race 
unless  the  voltage  is  very  high  or  the  field  very  weak. 
A  shunt  motor's  speed  will  increase  about  20  <f,  upon 
the  removal  of  load,  but  not  more  because  its  field  is 
independent  of  the  armature.  On  the  other  hand  a 
series  motor  races  badly  because  the  field  decreases 
as  the  armature  current  decreases,  and  the  higher  the 
speed  the  weaker  does  the  field  become,  until,  if  un- 
checked, the  armature  may  wreck  itself. 

This  peculiarity  may  be  profitably  followed  out  more 
closely.  Let  us  suppose  that  the  impressed  E.  M.  F.  is 
from  a  compound-wound  generator  and  remains  con- 
stant, and  that  the  motor  carrying  a  certain  load  has  a 
terminal  E.  M.  F.  of  E  volts,  distributed  between  its 
C.  E.  M.  F.  and  its  internal  resistance  R  :  or,  E  —  e  -+- 
I  R.  Now  let  part  of  the  load  be  removed;  a  less 
amount  of  energy  is  now  required  to  carry  the  decreased 
load.  The  amount  taken  by  the  motor  is  measured  by 


MOTOR     TESTING.  479 

the  product  E  I,  and  since  E  is  constant,  /  must  de- 
crease. As  /decreases,  the  drop  through  armature  and 
field  decreases;  now  since  I  R  has  decreased,  the  C.  E. 
M.  F.  must  increase,  for  E  —  c  -\-  f  R  is  a  constant. 
TheC.  E.  M.  F.  can  be  increased  in  two  ways:  by  strength- 
ening the  field,  by  raising  the  speed:  but  in  this  case  the 
field  is  weakened  as  /grows  less,  and  hence  the  speed  in- 
creases at  a  rapid  rate  to  compensate  for  the  weakened 
field  and  the  decreased  drop.  As  the  load  is  further 
lessened,  the  speed  soon  passes  the  safety  point  and  the 
belt  flies  off.  The  armature  unhindered  speeds  on,  its 
current  becoming  a  minimum,  and  its  C.  E.  M.  F. 
approaching  the  impressed  E.  M.  F.  as  a  limit.  The 
less  the  machine's  internal  resistance,  the  greater  its 
C.  E.  M.  F.  and  the  smaller  the  internal  drop. 

It  may  be  at  first  puzzling  to  find  a  way  to  check  the 
series  machine's  tendency  to  race:  In  the  equation  E  — 
c  A-  I  R  we  see  that  if  /  R  can  be  made  large,  Es  value 
may  be  correspondingly  small,  also  if  R  be  made  large,  the 
drop  /  R  will  be  large,  and  without  any  abnormal  value 
of  /.  To  secure  this  the  starting  box  is  thrown  into 
series  connection  with  the  motor,  and  is  gradually  cut 
out  as  the  device  in  operation  gains  headway.  It  is  thus 
that  the  speed  is  held  within  proper  limits. 

In  a  shunt  motor  the  field  and  the  armature  circuits 
are  in  multiple,  so  that  if  the  applied  voltage  is  main- 
tained constant  the  current  which  each  gets  depends 
solely  upon  its  effective  resistance.  The  field  circuit 
containing  only  ohmic  resistance  its  current  remains 
constant,  excepting  in  so  far  as  this  resistance  may  be 
increased  by  heat.  The  armature  when  at  rest  also 
contains  only  ohmic  resistance,  and  this  being  very  low 


480  TESTING    OF    DYNAMOS    AND    MOTORS. 

would  admit  an  abnormal  current  were  no  steps  taken  to 
prevent  it.  The  starting  box  is  therefore  used  to  limit 
the  current  value  at  starting.  As  the  armature  begins  to 
turn  it  generates  a  C.  E.  M.  F.,  which  may  replace  the 
ohmic  resistance  of  the  box;  as  the  speed  rises  the 
C.  E.  M.  F.  does  also,  and  the  box  is  gradually  cut  out, 
until  the  C.  E.  M.  F.  is  finally  adequate  to  control  the 
armature  current. 

When  full  load  is  on  a  shunt  motor  and  the  armature 
current  reaches  its  maximum  value,  the  brushes  have 
their  greatest  backward  lead,  and  it  is  here  that  the 
armature  reaction  is  greatest,  and  weakens  the  field 
most.  If  at  this  point  the  load  be  suddenly  removed, 
the  first  effect  is  to  strengthen  the  field  by  withdrawing 
the  armature's  reaction,  and  secondly  to  lessen  the  drop 
by  decreasing  /.  The  first  effect  tends  to  lower  the 
speed  and  the  latter  to  raise  it,  with  the  result  that  the 
speed  rises  about  20  $. 

The  difference  between  shunt  and  series  motors  in  this 
respect  is,  that  on  series  motors  the  increase  of 
C.  E.  M.  F.  due  to  increase  of  speed  which  follows  the 
removal  of  load  weakens  the  field,  while  on  shunt 
motors  the  reverse  is  true.  Another  difference  is  in  the 
effect  of  heating.  On  a  series  machine  at  constant 
voltage  and  a  given  current  the  speed  is  higher  at  the 
beginning  than  at  the  end  of  a  heat  test,  because  the 
increased  internal  resistance  raises  the  drop,  thereby 
lowering  the  effective  impressed  voltage,  and  as  the  speed 
depends  upon  this,  it  decreases  with  it.  On  a  shunt 
machine,  the  initial  speed  is  lower  than  the  final,  because 
as  the  shunt  field  heats  its  increased  resistance  admits  less 
current,  thereby  weakening  the  field  and  raising  the  speed. 


MOTOR     TESTING.  481 

The  fact  that  the  field  and  armature  of  a  series  motor 
are  in  series  adapts  it  for  use  where  jarring  is  excessive 
or  where  the  external  circuit  includes  a  sliding  or  rolling 
contact.  If  the  circuit  is  momentarily  interrupted,  field 
and  armature  are  alike  deprived  of  current,  and  when  the 
circuit  is  closed  the  same  current  flows  through  both,  so 
that  a  prolonged  short  circuit  is  impossible  unless  the 
rotation  of  the  armature  is  prevented.  If  the  circuit  of 
a  shunt  motor  is  broken  for  a  moment,  the  motor  loses 
its  field,  and  when  the  circuit  is  again  Closed  the  arma- 
ture short  circuits  the  field  and  admits  an  abnormal  cur- 
rent, which  not  only  gives  a  violent  wrench  to  the  belt  or 
gearing,  but  also  endangers  the  life  of  the  armature. 
The  result  of  these  facts  is  that,  as  a  rule,  portable 
motors  such  as  are  used  on  street  railroads  and  the  like 
are  series  motors;  while  stationary  motors  are  generally 
shunt-wound. 

T  EST  XVIII.  Series  Motor-  Generator  Test. — O  n  ace o  u  n  t 
of  complications  which  arise  due  to  the  fact  that  series 
machines  support  a  field  only  on  closed  circuit,  and  that 
generators  and  motors  run  in  opposite  directions  forgiven 
connections,  special  precautions  must  be  taken  in  running 
a  series  motor-generator  test.  The  loss  is  best  supplied 
by  an  engine  or  by  a  motor  whose  speed  can  be  easily 
controlled.  The  following  scheme  is  adapted  to  testing 
street  railway  motors  where  made  in  quantities,  and 
when  it  is  necessary  to  use  as  little  power  as  possible 
from  the  engine.  Although  not  in  general  use  the  plan 
has  commendable  features.  A  and  B,  Fig.  158,  are  two 
motors  geared  to  axle,  6",  which  is  belted  to  engine,  E, 
and  turns  always  in  the  same  direction.  A  is  connected 
to  generate  for  this  direction  of  rotation,  and  B  is  con- 


482 


TESTING    OF    DYNAMOS    AND    MOTORS. 


nected  so  that  it  will  not  generate.  It  will  therefore  run 
as  a  motor,  and  in  the  same  direction  as  the  axle.  To 
run  the  test  with  both  motors  connected  alike,  it  is  nec- 
essary either  to  have  an  intermediate  gear  on  one  motor, 
or  to  have  the  motors  run  on  opposite  sides  of  the  same 
gear,  so  that  the  armatures  will  turn  in  opposite  direc- 
tions. Usually  the  best  method  is  to  reverse  the  con- 


=CH 


-m-  -| 

B 

=D> 

A 

FIG.   158. 

nections  and  leave  the  direction  of  rotation  the  same. 
A  and  B  have  their  fields  and  armatures  in  series  as 
shown  in  the  diagram  and  include  in  circuit  a  switch  K 
and  a  variable  resistance,  R,  capable  of  carrying  the 
machine's  full  current.  R  exceeds  the  critical  resistance 
of  the  machine  for  the  given  speed,  so  that  upon  closing 
K  the  dynamo  will  not  generate  until  part  of  R  is  cut 
out.  Before  starting  a  test  for  the  first  time  it  is  well  to 
preliminarily  determine  the  correct  connections  for  the 
dynamo  to  generate.  This  is  done  by  connecting  the 
machine  as  shown  in  Fig.  159  and  bringing  it  up  to 
speed.  In  Fig.  159  R  is  the  starting  box,  al  #3  the 


MOTOR     TESTING. 


483 


f 

A 

f. 

|R 

,K 

_1«.           \ 

oJ 

armature  terminals,  and  /,  /a  the  field  terminals.  If  now 
the  machine  refuses  to  generate  upon  closing  K  and 
slowly  working  R  out,  the  armature  leads  should  be 
interchanged. 

There  is  also  a  question  of  speed  to  be  decided 
before  starting.  Running  at  its  rated  current  and 
speed,  a  machine  as  a  generator 
will  not  produce  an  E.  M.  F.  as 
great  as  the  impressed  would 
have  to  be  in  order  to  run  the 
machine  as  a  motor  under  simi- 
lar conditions,  and  hence  the 
system  is  not  subjected  to  as 
severe  a  test  as  it  ought  to  be. 
To  compensate  for  this,  the 
gearing  or  belting  should  be 
arranged  to  run  A  above  its 

rated  speed  and  thus  raise  the  E.  M.  F.  to  its  rated 
value.  The  necessary  increase  in  speed  can  be  approxi- 
mately calculated  if  A's  internal  resistance  is  known. 
Let  the  machine's  internal  resistance  be  r;  then  i  r  is  the 
internal  drop  when  current,  /",  flows;  let  the  E.  M.  F. 
desired  to  be  impressed  at  the  motor  terminals  be  E. 
Then  will  the  C.  E.  M.  F.  be  E  —  i  r  =  e.  Let  n  be  the 
speed  of  A  running  as  a  motor  with  an  impressed 
E.  M.  F.  of  E  volts  and  a  current  of  /'  amperes.  Since 
as  a  motor  the  machine  generates  a  C.  E.  M.  F.  of  c 
volts,  and  as  a  dynamo  it  must  generate  an  E.  M.  F.  of 
E  volts,  we  may  write,  e  :  E  \\  n  :  n',  where  «'  is  the 
required  speed.  This  gives  us 

n  E          n  E 


FIG.  159. 


n   = 


E-ir' 


484  TESTING    OF    DYNAMOS    AND    MOTORS. 

where  all  the  terms  are  known  and  hence  ;/'  deter- 
mined. 

On  account  of  the  series  machine's  ability  to  rapidly 
pick  up  as  soon  as  it  begins  to  generate,  it  is  well  to  pro- 
vide belt  guards  to  avoid  the  annoyance  of  losing  the 
belt  under  sudden  overloads.  A  further  precaution  is  to 
insert  a  light  fuse  at  the  start,  and  then  cut  it  out  when 
the  test  is  under  way.  If  the  motor  shaft  is  arranged  to 
be  thrown  in  by  a  clutch  the  start  is  much  smoother. 
In  starting  up,  the  machine  is  brought  up  to  speed,  K 
closed,  and  R  slowly  worked  out,  at  the  same  time  weak- 
ening B's  field  by  means  of  the  shunt  r  shown  in  Fig. 
158.  As  soon  as  the  ammeter  shows  A  to  be  generating 
R  must  be  very  carefully  handled  to  avoid  precipitating 
a  heavy  overload  and  throwing  the  belt.  A  will  refuse 
to  generate  until  a  certain  amount  of  R  has  been  cut  out, 
and  will  than  pick  up  very  rapidly.  It  is  absolutely 
necessary  that  means  be  provided  for  weakening  the 
motor  field,  otherwise  since  the  same  current  must  pass 
through  both  machines,  and  since  they  run  at  the  same 
speed,  the  C.  E.  M.  F.  of  B  will  be  the  same  as  the 
E.  M.  F.  of  A,  and  a  load  cannot  be  worked  on.  The 
shunt  affords  the  same  regulation  as  obtains  on  a  car, 
but  has  a  different  relation,  in  that  on  a  car  its  value  is 
constant  and  the  speed  variable,  while  in  this  test  the 
shunt  is  variable  and  the  speed  constant. 

A's  current  passing  through  B  runs  it  as  a  motor,  and 
helps  to  turn  the  system,  thus  lessening  the  demand  on 
the  supplier,  which  then  supplies  only  energy  enough  to 
cover  the  losses,  which  may  amount  to  from  25$  to  35$ 
of  the  motor's  output.  After  running  A  for  a  stated 
time  as  a  dynamo  it  is  changed  over  and  run  as  a  motor. 


MOTOR     TESTING. 


485 


This  change  is  most  rapidly  effected  by  using  a  crossed 
belt  to  reverse  the  direction  of  rotation;  it  is  then  only 
necessary  to  move  the  shunt  from  B  to  A. 

TEST  XIX.  Testing  Series  Machines  with  Water  Rheostat. 
—In  Fig.  160  are  shown  connections  for  a  second 
method  of  testing  series  machines.  It  is  a  modified  form 
of  motor-generator 
test,  but  differs  from 
it  in  that  the  dyna- 
mo's energy  is  not 
returned  to  the  mo- 
tor. The  amount 
of  energy  supplied 
must  be  something 
over  the  rated  out- 
put of  one  machine. 
G  is  a  generator  of 
the  same  voltage  as  the  machines  under  test,  D  is  the 
dynamo  armature,  M,  that  of  the  motor;  F  and  F'  the 
dynamo  and  motor  fields  respectively.  It  will  be  seen 
that  F,  F\  and  J/,  are  in  series,  and  that  F  is,  therefore, 
excited  from  G.  W  is  a  water  rheostat  to  which  Z>'s 
terminals  are  connected:  A' is  a  switch  across  which  is 
the  starting  box  R.  M  and  D  are  geared  or  bolted  to 
the  same  shaft  or  to  each  other  and  hence  start  up  to- 
gether. When  R  is  all  cut  out  it  is  short  circuited  by 
means  of  K-,  if  the  speed  is  then  too  high  and  the  load  is 
low,  the  plates  of  the  water  rheostat  must  be  brought 
nearer  together,  thereby  decreasing  its  resistance,  in- 
creasing the  load,  and  lowering  the  speed.  If  the  load  is 
apparently  all  right  and  the  speed  still  high,  the  am- 
meter or  voltmeter  may  be  "off."  Only  frequent  cali- 


FIG  160. 


486  TESTING    OF    DYNAMOS    AND    MOTOKS. 

bration  can  eliminate  liability  to  this  error.  If  the  load 
is  certainly  all  right  and  the  speed  high,  the  indications 
are  of  a  weak  field,  which  may  be  due  to  short  circuited 
turns  in  a  field  coil;  to  reversal  of  a  field  spool,  on  any 
motor  having  more  than  two  coils;  to  a  loose  joint  in  the 
magnetic  circuit;  to  an  abnormally  wide  air  gap;  to  an 
inferior  quality  of  iron  in  the  armature  core  or  frame; 
or  to  displacement  of  brushes.  Whether  there  are  any 
short  circuited  field  turns  or  not  can  be  decided  by  pass- 
ing current  through  all  and  taking  the  drop  on  each. 
If  one  coil  is  reversed  on  a  motor  having  but  two  coils, 
it  will  not  start  at  all.  If  one  of  four  is  reversed  it  is 
readily  located  by  trying  to  start  the  motor  using  but  two 
coils  at  a  time.  When  the  pair  in  use  are  of  proper 
polarity  the  motor  will  start,  though  more  current  will 
be  required  than  ordinarily  to  do  so.  When  of  the 
wrong  polarity  no  safe  current  will  start  the  motor.  We 
emphasize  safe  because  it  is  a  fact  that  sufficient  current 
through  the  armature  would  enable  it  to  react  upon  the 
pole  pieces,  and  produce  field  enough  to  start.  Once 
started,  the  armature  would  race.  The  loose  joint  and 
air  gap  difficulty  can  best  be  located  by  inspection,  while 
the  question  of  inferiority  of  iron  can  be  considered 
after  all  other  probable  difficulties  have  been  eliminated. 
If  there  are  no  facilities  for  magnetic  testing,  we  must 
resort  to  the  somewhat  crude  method  of  interchanging 
the  parts  of  the  faulty  motor  with  those  of  one  that  we 
know  to  be  all  right. 

A  modification  of  the  above  test  consists  in  running  D 
as  a  self-exciting  series  dynamo,  with  the  advantage 
that  a  greater  E.  M.  F.  is  applied  to  M,  because  there 
is  no  impressed  voltage  lost  in  the  resistance  of  D's  field, 


MOTOR     TESTING.  487 

and  this  may  be  considerable.  In  this  test  the  connec- 
tions depend  upon  the  direction  of  rotation.  If  with 
given  connections  D  refuses  to  pick  up,  the  direction  of 
rotation  must  be  reversed.  Of  course  with  separate  ex- 
citation the  direction  of  rotation  need  not  be  considered. 
When  the  dynamo  does  "pick  up"  it  does  so  with  a 
rush  and  the  speed  drops  rapidly.  This  effect  is  more 
marked  in  warm  weather  than  in  cold,  for  the  water  box 
resistance  is  lower.  On  the  other  hand  the  machine  may 
refuse  to  generate  for  want  of  a  sufficiently  strong  re- 
sidual field.  In  this  case  the  practice  is  to  short  circuit 
the  leads  with  a  piece  of  fuse  wire  in  multiple  with  the 
water  box.  As  soon  as  the  machine  generates  it  blows 
the  fuse,  and  then  continues  to  send  current  through  the 
water  box,  in  virtue  of  the  strong  field  provided  by  the 
short  circuit.  The  above  test  method  is  much  used  in 
testing  street  railway  motors,  and  for  this  purpose  a  box 
5  feet  x  il/z  feet  -j-  i^  feet  is  sufficient  to  carry  the 
load. 

TEST  XX.  Efficiency  Test. — A  modification  of  Test 
XVIII.  is  used  for  determining  the  efficiency  of  series 
motors.  In  Fig.  158  shaft  5",  instead  of  being  driven 
by  the  engine  £,  is  driven  by  a  motor  which  we  will 
call  M.  The  test  is  made  in  the  following  steps: 
i.  The  energy  necessary  to  turn  M  free  at  the  proper 
speed  is  taken.  2.  M  is  then  belted  to  S  and  all  pin- 
ions on  A  drawn.  The  energy  required  by  M  to  run 
A's  armature  at  proper  speed  is  then  measured.  3.  A's 
pinion  is  replaced  and  £'s  removed  and  readings  taken 
on  M.  4.  B  is  geared  to  A  in  the  usual  way,  and  the 
energy  again  measured.  These  successive  measure- 
ments give  the  data  for  finding  the  frictional  losses  on 


488 


TESTING    OF    DYNAMOS    AND    MOTORS. 


all  moving  parts.  A  is  then  connected  as  a  generator 
and  runs  B  as  a  motor.  Readings  are  taken  as  in  former 
efficiency  tests  and  from  these  readings  the  efficiency  is 
deduced. 

TEST  XXI.    Efficiency    Test    with    Prony     Brake. — An- 
other good  way  of  finding  efficiency  is  by  means  of  the 


DETAILS  OF  E 


FIG.  161. 

Prony  brake,  a  device  much  used  in  testing  rooms.  One 
form  used  is  shown  in  Fig.  161,  where  M  is  an  iron 
pulley  keyed  to  the  armature  shaft.  About  the  outside 
of  this  pulley  is  placed  a  steel  strap,  one  of  whose  ends 
fastens  to  the  lower  arm  of  clamp  E,  the  other  end  to 
the  upper  arm  of  E  and  ending  at  Z>,  where  the  weight 
hanger  W  is  attached.  The  grip  of  the  strap  can  be 
varied  at  will  by  adjustment  of  cord  /.  £Falso  is  vari- 


MOTOR     TESTING.  489 

able.  The  inner  rim  of  the  pulley  is  flanged  and  will 
hold  a  considerable  amount  of  water,  which,  poured  in 
after  the  wheel  is  in  motion,  is  kept  in  by  the  centrifugal 
force,  and  serves  to  cool  the  rims.  When  the  wheel 
stops  a  large  portion  of  this  water  is  ejected,  so  that  the 
operator  must  be  on  the  lookout.  The  action  of  the  brake 
is  as  follows:  The  motor  armature  turns  left  to  right 
and  tends  to  carry  the  mass  IV  around  with  it  as  soon 
as  the  steel  strap  is  tightened.  On  the  other  hand,  }V 
resists  by  its  weight  the  tendency  to  be  carried  around. 
To  measure  the  load  on  the  motor,  the  weights  on  Wand 
the  tension  on  E  are  so  adjusted  that  the  weight  is  bal- 
anced exactly  at  the  horizontal  line;  the  weight  pulls 
on  the  wheel  with  the  leverage  OT.  The  work  done 
when  a  balance  is  effected  is  measured  in  mechanical  units 
by  taking  the  product  of  the  circumference  of  the  wheel, 
the  number  of  revolutions  per  second,  and  the  weight  W. 
If  d  be  the  diameter  of  the  wheel  the  circumference  is 
3.1415  d  or  7t  d.  The  power  or  work  per  second  then  is 


Power  =  7t  d  n  W, 


where  n  is  the  revolutions  per  second,  and  W  the  weight 
on  the  hanger.  The  intake  is  measured  electrically, 
and  is  the  product  E  x  /,  and  is  measured  in  watts. 

If  d\s  given  in  centimetres  and  Win  grams,  then  the 
output  is  measured  in  ergs  per  second,  and  the  watt  is 
equal  to  10,000,000  ergs  per  second:  therefore  we  have 

Output  in  watts  =  -  -  =  W0 , 

10,000,000 

also 

Intake  in  watts  =  E  x  I  —  W^\ 


490  TESTING    OF    DYNAMOS    AND    MOTORS. 

we  then  have 

n  d  nw 


_„-   .  0 

Efficiency  =  -=~  = 


10,000,000  E  X  / 
If  d  is  given  in  feet  and  W  in  pounds  we  then  have 

Output  in  watts  =  1.36  it  d  W, 

and 

1.36  n  d  W 


Efficiency  = 


E  X 


The  advantages  of  this  method  are  that  the  apparatus  is 
compact  and  simple,  and  the  readings  easily  taken.  The 
drawback  is  that  the  energy  is  entirely  lost,  being  con- 
sumed in  friction  on  the  pulley  and  strap.  If  the  test  is 
long  continued  the  rise  of  temperature  of  the  pulley  will 
cause  the  water  in  the  rim  to  rise  to  the  boiling  point. 

In  testing  series  machines  rigidly  connected  it  is  a 
wise  precaution  to  have  the  scheme  of  wiring  embodied 
in  a  drawing,  which  can  be  followed  in  connecting  up 
each  set  of  motors.  This  drawing  should  include,  not 
only  the  motors  themselves,  but  the  controller  and  instru- 
ments as  well.  Then  if  a  controller  should  have  a 
wrong  connection,  or  if  the  polarity  of  armature  or 
field  is  reversed,  through  some  error  in  winding,  or  in 
bringing  out  the  leads,  this  fact  is  likely  to  be  discovered 
by  a  reversal  in  the  direction  of  rotation,  or  by  a  refusal 
to  run  at  all. 

Having  discovered  that  an  error  of  this  kind  exists 
somewhere  in  the  set  under  test,  the  next  step  is  to 
locate  it.  Since  there  are  two  motors  concerned  it  is 
highly  likely  that  one  of  them  is  correct,  and  a  compari- 


MOTOR     TESTING.  49! 

son  of  the  wiring,  when  the  machines  are  made  to  run, 
with  the  diagram,  will  identify  the  faulty  machine. 

If  no  knowledge  exists  as  to  the  relation  of  direction 
of  rotation  and  connections,  so  that  it  is  not  known  when 
the  machine  is  running  correctly,  the  best  procedure  is 
to  connect  up  and  run  a  motor  which  has  been  tested, 
and  is  known  to  be  correct.  In  this  case  the  motor  in 
the  rack  which  runs  like  it  is  all  right,  and  the  other  one 
probably  at  fault.  To  make  perfectly  sure,  however,  the 
test  motor  must  be  run  on  both  sets  of  wires,  then  if  its 
direction  of  rotation  is  the  same  on  both  sets  of  wires,  it 
locates  the  latter  set,  with  its  motor,  as  the  faulty  one. 
If  the  rotation  of  the  test  motor  is  reversed  in  passing 
from  one  set  to  the  other,  it  shows  that  the  motors  are 
right  and  the  trouble  is  in  the  controller. 

Having  now  identified  the  faulty  machine  we  must  de- 
termine whether  the  trouble  is  in  the  armature  or  fields. 
The  best  way  to  do  this  is  to  place  an  armature  which  is 
known  to  be  correct  in  the  frame,  and  to  run  it;  if  the 
two  run  in  the  same  direction  it  shows  that  they  are  alike 
and  that  the  fields  are  at  fault.  The  methods  of  testing 
out  faults  in  armature  and  field  have  already  been  given 
in  earlier  chapters,  and  will  not  here  be  repeated. 

In  such  tests  it  simplifies  matters  if,  instead  of  a  con- 
troller, with  its  complicated  wiring,  an  ordinary  rheostat 
is  used  as  a  starting  box.  The  motors  can  then  be  con- 
nected as  shown  in  Fig.  162,  where  //'  are  the  terminals 
of  the  starting  box.  In  connecting  the  two  motors  cor- 
responding field  and  armature  terminals  go  together, 
leaving  the  same  leads  on  the  two  motors  to  be  connected 
to  the  box.  It  then  makes  no  difference  which  lead  is 
connected  to  /  or  /'  of  the  box,  for,  since  both  field  and 


492 


TESTING    OF    DYNAMOS   AND    MOTORS. 


jwwmi 


armature  are  included  between  the  leads,  the  direction  of 
rotation  will  be  the  same  for  either  connection. 

The  compound-wound  motor  combines  the  general 
characteristics  of  both  the  series  and  of  the  shunt  motor. 
A  dynamo  compounded 
for  constant  potential,  if 
differentially  connected, 
will  as  a  motor  automati- 
cally regulate  for  con- 
stant speed.  By  con- 
necting differentially  is 
meant  that  the  series 
field  opposes  the  shunt 
field  in  its  magnetizing 
effect.  Supposing  a  com- 
pound-wound motor  to 
be  running  at  its  proper 
speed,  and  let  us  suppose 

the  load  to  be  increasing,  the  tendency  of  the  added  load  is 
to  cause  the  speed  to  drop,  and  it  will  momentarily  do  so. 
With  this  fall  of  speed  comes  a  decrease  in  the  C.  E.  M. 
F.,  and  the  current  in  the  armature  rises,  and  with  it  the 
strength  of  the  series  field;  but  the  series  field  opposes 
the  shunt  field  and  hence  the  field  of  the  motor  is  weak- 
ened, and  the  speed  increases. 

In  connecting  a  compound-wound  motor  differentially 
the  connections  can  be  tested  as  follows:  With  the  given 
connections  the  armature  current  is  read,  and  the  speed 
noted.  The  connections  of  the  series  field  are  then  re- 
versed, and  the  armature  current  adjusted  to  the  same 
value  as  before  and  the  speed  taken.  The  differential 
connection  will  give  the  higher  speed,  as  it  gives  the. 


MOTOR     TESTING.  493 

weaker  field.  The  test  can  also  be  made  by  short  cir- 
cuiting the  series  field  so  that  the  motor  is  first  running 
on  the  shunt  field  alone,  and  reading  the  current  and 
speed  as  above.  The  series  coil  is  then  cut  in,  the  cur- 
rent adjusted,  and  the  speed  read.  In  this  case  the  ter- 
minal E.  M.  F.  will  have  to  be  raised  and  the  latter  speed 
will  be  found  to  be  lower  than  the  first,  if  the  connections 
are  differential,  but  the  fall  of  speed  will  not  be  so  great 
as  when  cumulatively  connected.  If  instead  of  keeping 
the  current  constant  the  E.  M.  F.  is  constant,  the  differ- 
ential connection  will  give  a  higher  speed  than  with  the 
shunt  field  alone,  while  the  cumulative  connection  will 
give  a  lower  speed.  The  statement  above  that  the  differ- 
ential connection  gives  a  lower  speed  than  the  shunt  field 
alone  may  at  first  seem  contradictory.  If  we  remember, 
however,  that  when  the  E.  M.  F.  is  raised  to  compensate 
for  the  /  R  drop  in  the  series  coil,  the  current  in  the 
shunt  field  is  thereby  increased,  and  that  the  shunt  field 
is  relatively  stronger  than  the  series  field,  we  see  that  in 
this  case  the  motor  field  is  strengthened,  and  hence  the 
speed  falls.  With  the  cumulative  connection  the  field  is 
strengthened  to  a  greater  extent,  and  the  fall  in  speed  is 
greater. 

In  testing  rooms  it  is  common  practice  to  run  com- 
pound-wound motors  with  the  fields  cumulatively  con- 
nected, and  in  motor-generator  tests  this  connection  is 
necessary  unless  there  is  a  heavy  shunt  on  the  motor 
series  field  to  weaken  its  effect.  In  the  motor-generator 
tests  already  cited  it  was  found  impracticable  to  get  the 
load  on  slowly  when  the  fields  were  connected  in  opposi- 
tion, and  it  would  be  only  theoretically  possible  with  the 
shunt  field  rheostats  so  arranged  as  to  gradually  intro- 


494  TESTING    OF    DYNAMOS    AND    MOTORS. 

duce  resistance  into  the  shunt  field  circuit.  Under  these 
conditions  the  system  would  be  very  unstable,  and  the 
least  variation  in  speed  precipitates  an  overload  or  even 
a  short  circuit. 

To  render  the  control  in  putting  on  a  load  more 
complete,  it  is  sometimes  necessary  to  use  a  shunt  board 
on  the  series  field  of  the  cumulatively  connected  motor: 
this  is  the  case  where  the  movement  of  the  rocker  arm 
is  too  limited  to  admit  of  working  on  the  load  by  shifting 
the  brushes.  The  shunt  board  is  also  needed  when  the 
shunt  field  has  been  so  weakened  that  the  machine  is 
practically  running  as  a  series  motor.  This  condition  is 
generally  indicated  by  a  steady  uncontrollable  sparking 
at  the  brushes,  which  is  only  relieved  by  strengthening 
the  shunt  field.  The  reason  for  this  lies  in  the  fact  that 
if  the  motor  is  running  as  a  series  motor  the  neutral 
point  is  not  the  same  as  under  normal  conditions,  and 
may  have  shifted  out  of  the  range  of  the  movement  of 
the  rocker  arm.  A  trial  is  the  best  test  of  the  necessity 
of  a  shunt  board. 

We  see  then  that  there  are  two  conditions  under  which 
a  shunt  board  can  be  profitably  used.  i.  With  fields 
cumulatively  connected,  where  the  shunt  board  weakens 
the  assisting  power  of  the  series  coils  and  does  away  with 
sparking.  2.  Where  the  fields  are  differentially  con- 
nected and  the  opposing  power  of  the  series  field  is  to  be 
weakened. 

The  series  motor  property  of  the  compound-wound 
motor  makes  it  safer  to  break  its  shunt  field  under  full 
load  than  under  light  load,  provided  the  fields  are 
cumulatively  connected.  In  this  case  at  full  load  the 
current  in  the  series  winding  is  large  and  the  field  is 


MOTOR     TESTING.  495 

mainly  due  to  it,  so  that  the  withdrawal  of  the  shunt 
field  will  have  no  further  effect  than  to  increase  the 
sparking,  and  perhaps  raise  the  speed  somewhat.  At 
light  load  the  field  may  be  considered  as  entirely  due  to 
the  shunt  coils,  and  the  series  field  is  very  small,  so  that 
to  break  the  former  would  precipitate  a  short  circuit. 
If  the  armature  does  not  burn  out  the  motor  will  start  to 
racing  and  throw  its  belt.  If  while  the  armature  is  at  a 
high  speed  the  shunt  field  be  again  closed,  the  motor  will 
generate  a  higher  C.  E.  M.  F.  than  the  impressed  E.  M.  F. 
and  will  momentarily  reverse  the  dynamo  that  is  running 
it.  This  action  lasts  only  so  long  as  the  momentum  of 
the  motor  enables  it  to  hold  its  speed,  and  the  system 
soon  returns  to  its  normal  state.  The  above  reactions 
are  especially  pronounced  where  the  machines  are  heavy 
and  strongly  overcompounded. 

If  both  machines  are  belted  to  the  same  countershaft 
the  above  reversals  cannot  take  place  until  the  motor 
belt  is  thrown,  as  neither  can  run  faster  than  the  engine 
does.  If  the  machines  are  simply  belted  together  and 
the  loss  supplied  from  a  dynamo  no  restriction  is  present, 
and  the  cycle  of  reactions  is  similar  to  several  that  have 
already  been  described. 

Compound-wound  motors  of  large  output  are  not  in 
general  use  because  for  most  machine  work  a  shunt  motor 
running  on  constant  potential  mains  regulates  sufficiently 
well. 

In  belting  a  motor  to  a  shafting  attention  must  be  paid 
to  two  points,  i.  The  higher  the  speed  allowed  on  the 
motor  the  higher  will  be  the  efficiency.  2.  The  tools 
operated  by  the  shafting  must  be  run  at  a  speed  sufficient 
to  secure  a  high  shop  efficiency.  The  designed  speed  of 


496  TESTING    OF    DYNAMOS    AND    MOTORS. 

most  types  of  commercial  motors  is  stamped  on  the  name 
plate,  and  is  the  speed  at  which  the  motor  should  run  at 
full  load. 

It  sometimes  happens  that  it  is  desired  to  add  tools 
to  a  line  of  shafting  that  already  fully  loads  the 
motor  running  it.  It  is  possible  to  do  this  without  over- 
loading the  motor,  by  putting  a  larger  pulley  on  the 
countershaft  or  a  smaller  one  on  the  armature,  thereby 
decreasing  the  speed  of  the  countershaft,  the  motor 
armature  keeping  the  same  speed  as  before.  Since  now 
the  countershaft  is  running  at  a  slower  speed,  each 
machine  turns  slower  and  does  less  work  in  a  given  time, 
the  new  machines  making  up  the  difference.  It  is  thus 
seen  that  the  total  work  turned  out  is  the  same  as  before, 
since  the  load  on  the  motor  is  the  same  whether  it  carries 
a  few  machines  heavily  loaded  or  more  machines  less 
heavily  loaded. 

At  times  in  testing  rooms  it  is  desirable  to  throw  a 
load  on  to  a  line  of  shafting  greater  than  the  engine 
driving  the  shafting  is  able  to  carry.  In  this  case  a 
motor  may  be  belted  to  the  shafting,  and  run  from  a 
dynamo  attached  to  another  engine.  In  making  the 
motor  connections  one  must  be  careful  not  to  make  the 
motor  oppose  the  engine  as  regards  its  direction  of 
rotation.  On  self-excited  machines,  one  familiar  with  the 
rules  for  connections  can  predict  the  direction  of  rota- 
tion, and  act  accordingly;  but  if  the  motor  is  separately 
excited  no  rule  is  available  and  recourse  must  be  had  to  a 
preliminary  test.  To  insure  that  a  motor  attached  to  an 
engine  shall  concur  in  effort  with  the  engine,  the  motor 
fields  must  be  so  excited  that  the  E.  M.  F.  of  the  arma- 
ture is  opposed  to  that  of  the  dynamo  that  is  to  run  it. 


MOTOR     TESTING.  497 

It  then  comes  to  this,  that  since  the  E.  M.  F.  generated 
by  the  motor  armature  is  to  become  its  C.  E.  M.  F.  it 
must  oppose  the  impressed  E.  M.  F.  Having  secured 
this,  and  adjusted  the  fields  so  that  the  voltage  read 
across  the  switch  is  low,  the  switch  is  closed  and  the  field 
weakened.  The  motor  then  takes  load  and  relieves  the 
engine. 

In  belting  up  the  motor  for  such  work,  care  must  be 
taken  to  select  pulleys  of  such  a  size  that  countershaft 
and  motor  are  both  running  at  their  proper  speed.  Sup- 
posing the  motor  to  make  1,000  revolutions  and  the  shaft 
300,  if  the  diameter  of  the  pulley  is  4  feet,  that  of  the 
armature  pulley  can  be  found  as  follows:  The  circum- 
ference of  the  shaft  pulley  is  TT  Z>  or  3. 1416  x  4  which 
equals  12.56  feet,  and  in  one  minute  the  belt  travels 
12.56  x  300  =  3,768  feet.  Now  the  armature  pulley  must 
travel  at  the  same  rate,  and  if  d  be  its  diameter  we  have 
d  X  3.1414  X  1,000  =  3,768,  which  gives  us 

d  —  -         -  —  1.2  feet. 

The  general  formula  is 

D  X  TT  X  //  =  d  X  n  X  «', 
or 

d          n 


where  </and  D  are  the  diameters  and  ;/  and  n'  the  revo- 
lutions per  minute  of  the  shaft  and  armature.  In  the 
above  case  this  gives  us 

d__         300 
4          1,000 


498  TESTING    OF    DYNAMOS    AND    MOTORS. 

or 

rf=I^  =  I.zfeet. 

I,OOO 

This  method  of  relieving  an  overloaded  engine  is  fre- 
quently resorted  to,  and  sometimes  at  the  end  of  a  test 
the  engineer  is  surprised  to  find  that  he  is  unable  to  stop 
his  engine  by  shutting  off  the  steam.  In  shutting  down, 
the  motor  switch  should  be  opened  first.  It  may  even 
happen  that  the  motor  is  running  the  engine;  this  is  apt 
to  happen  when  two  dynamos  running  from  different 
engines  are  placed  in  multiple,  and  by  careless  handling 
of  a  rheostat  one  of  them  is  changed  over  into  a  motor. 

If  a  machine,  as  a  dynamo,  is  designed  to  give  a  cer- 
tain voltage,  it  will  not  as  a  motor  give  the  same  speed 
when  run  at  that  voltage,  but  will  give  a  speed  from  10  $ 
to  20  <f>  lower.  The  reason  for  this  is  as  follows:  In  the 
case  of  the  dynamo  the  losses  due  to  friction,  belt  ten- 
sion, armature,  and  field  resistance,  etc.,  are  looked  after 
by  the  engine  and  the  voltage  is  maintained  over  and 
above  these  losses.  In  other  words,  the  total  E.  M.  F. 
of  the  dynamo  is  greater  than  its  terminal  E.  M.  F.  In 
the  case  of  the  motor  the  losses  must  be  supplied  by  the 
energy  given  to  the  motor  electrically,  and  consequently 
the  voltage  available  for  producing  rotation  is  not  equal 
to  the  impressed  E.  M.  F.,  but  is  lower  than  this  by  an 
amount  equal  to  the  drop  through  the  motor.  Hence 
the  speed  of  the  motor  is  lower  than  that  of  the  dynamo. 

The  case  of  the  motor  is  not  then  the  exact  counter- 
part of  that  of  the  dynamo.  Thus,  if  the  output  of 
the  dynamo  be  25  amperes  at  100  volts,  the  output  is 
2,500  watts.  If  the  losses  cared  for  by  the  engine  are 


MOTOR     TESTING.  499 

equal  to  500  watts,  then  the  energy  involved  in  the 
dynamo  is  2,500  -(-  500  or  3,000  watts.  On  the  other 
hand,  if  the  motor  be  running  with  an  impressed  E.  M.  F. 
of  100  volts  and  is  taking  25  amperes,  and  if  the  losses 
in  the  motor  are  500  watts,  then  the  energy  effective  in 
producing  rotation  is  2,500  —  500  or  2,000  watts,  as 
against  3,000  in  the  dynamo.  In  order  then  that  the 
motor  shall  run  at  the  same  speed  as  the  dynamo,  its  total 
energy  of  rotation  must  equal  the  output  of  the  dynamo, 
and  we  have  2,500  -f-  500  =  3,000  watts  necessary  to  be 
supplied  to  the  motor  terminals.  To  do  this  the  im- 
pressed voltage  must  be  raised  from  100  to  120  volts. 
Since  TOO  is  17  %  less  than  120  the  speed  of  the  motor  at 
100  volts  will  be  17  $  below  that  of  the  dynamo,  on  the 
supposition  that  the  speed  varies  directly  as  the  im- 
pressed E.  M.  F. 

The  ratio  of  speeds  of  the  same  machine,  when  run  as 
a  dynamo  and  motor,  is  given  approximately  by  the 
fraction 


//• 


where  Jf7d  is  the  total  intake  of  the  dynamo,  and  Wm  is 
the  output  of  the  motor,  /.  e.,  the  energy  available  for 
producing  rotation  after  all  losses  are  deducted. 

Small  machines  which  are  successful  as  motors  do  not 
give  satisfaction  as  dynamos,  for  of  their  output  a  large 
part  is  consumed  in  energizing  the  magnetic  circuit. 
For  since  the  reluctance  of  the  magnetic  circuit  de- 
pends very  largely  upon  the  air  gap,  the  magnetic  circuit 
of  small  machines  is  of  such  high  reluctance  that  nearly 
or  quite  all  of  the  output,  of  the  machine  would 


500  TESTING    OF    DYNAMOS    AND    MOTORS. 

be  absorbed  by  the  fields.  On  the  motor,  energy  is 
furnished  for  exciting  the  fields  over  and  above  that 
constituting  the  rated  output  of  the  machine,  and  on 
small  motors  the  question  of  efficiency  is  not  so  much 
considered  as  that  of  getting  the  required  work  done; 
for,  since  the  entire  output  of  the  motor  is  small  the 
expense  is  not  greatly  increased  even  if  the  efficiency 
be  but  50  <f>  or  less. 

Where  a  considerable  amount  of  work  is  to  be  done  the 
-question  of  efficiency  and  of  the  number  of  machines 
involved  is  of  importance.  It  is  an  advantage  to  run  one 
machine  at  full  load  rather  than  two  at  half  load,  for  the 
efficiency  is  highest  at  full  or  overload;  also  the  fric- 
tional  losses  are  less  the  less  the  number  of  machines 
in  use.  To  eliminate  this  evil  as  much  as  possible, 
power  plants  are  divided  into  large  and  small  units. 
When  the  load  is  very  light  a  single  small  machine  is 
placed  in  service  and,  as  the  load  increases  beyond  its 
capacity,  larger  machines  take  its  place,  so  long  as 
a  single  machine  can  carry  the  load.  The  machines  are 
so  handled  that  all  machines  in  service  are  running  from 
three-fourths  to  full  load. 

Motors  are  run,  in  common  practice,  in  series  and  in 
multiple,  and  it  is  possible,  though  not  usual,  to  run 
them  in  series-multiple  if  proper  precautions  are  taken. 
In  general,  it  may  be  said  that  shunt  and  compound- 
wound  motors  are  run  in  multiple,  as  in  the  case  of 
factories  or  of  isolated  plants  on  lighting  mains;  and 
series  motors  in  both  multiple  and  series  as  in  street-car 
work.  In  testing  rooms  separate  excitation  is  also 
resorted  to  where  special  advantages  are  sought  for. 

When    running  motors    in  series    the  prime   requisite 


MOTOR     TESTING.  501 

is  that  their  armatures  be  rigidly  connected,  as,  for 
example,  where  all  are  belted  to  the  same  countershaft, 
or  when  two  motors  are  geared  to  the  car  axles.  To  see 
the  necessity  of  this  let  us  consider  the  case  of  two 
shunt  motors  connected  in  series  and  run  from  a  con- 
stant potential  dynamo,  but  without  any  mechanical  con- 
nection with  each  other.  If  both  motors  have  a  switch, 
one  may  be  closed  and  a  starting  box  placed  across  the 
other.  As  the  box  is  cut  out  only  that  motor  will  start 
across  whose  switch  the  box  is  placed,  for  this  motor 
alone  possesses  a  field.  Should  the  motors  be  separately 
excited  or  be  series  wound,  both  may  start,  and  that  one 
will  start  first  whose  frictional  resistance  and  inertia  is 
the  least,  both  being  started  without  load.  If  load  is 
now  placed  on  either  one  it  will  immediately  slow  down 
and  stop,  while  the  speed  of  the  other  one  will  rise.  If, 
instead  of  placing  the  box  across  one  switch,  both 
switches  are  closed,  and  the  box  is  in  series  with  the 
system,  then  upon  cutting  it  out  that  machine  will  start 
whose  friction  is  least  and  whose  field  is  the  strongest. 
Both  fields,  however,  will  be  very  weak,  and  only  one  of 
the  motors  will  take  a  load.  The  effect  of  placing  the 
box  in  this  last  manner  is  to  reduce  the  E.  M.  F.  applied 
to  the  fields  as  well  as  to  the  armatures,  thereby  greatly 
reducing  the  starting  power  of  the  motors.  Lastly, 
both  motors  can  be  started  by  using  two  starting 
boxes,  which  are  worked  simultaneously  or  by  con- 
necting the  box  so  as  to  span  both  switches.  But 
while  both  machines  will  start  one  will  stop  as  soon  as 
the  load  is  put  on. 

These  somewhat  curious  actions  depend  upon  the  dis- 
tribution of  the  impressed   E.   M.  F.  between  the  two 


502  TESTING    OF    DYNAMOS    AND    MOTORS. 

machines,  and  can  be  readily  accounted  for  as  follows: 
An  E.  M.  F.  impressed  upon  a  circuit  is  distributed 
along  the  circuit  according  to  the  ohmic  resistance  and 
the  C.  E.  M.  Fs.  present  in  the  circuit.  With  the  two 
motors  in  series  the  ohmic  resistance  of  the  circuit  is 
small  and  may  be  considered  as  uniformly  distributed; 
the  C.  E.  M.  Fs.  are  therefore  the  controlling  factors. 
In  the  first  case  considered,  one  motor,  which  we  will 
call  A,  has  its  switch  closed,  and  the  resistance  from 
terminal  to  terminal  being  very  low,  the  potential  differ- 
ence (measured  by  /  7?)  is  also  small;  also  since  the 
armature  short  circuits  the  fields,  they  cannot  acquire 
strength.  The  other  motor,  B,  having  an  open  switch 
across  which  the  box  is  placed,  its  potential  difference 
from  terminal  to  terminal  is  comparatively  high;  so  that 
a  strong  field  is  obtained  and,  as  the  box  is  cut  out,  the 
motor  easily  starts.  As  the  speed  rises  the  C.  E.  M.  F. 
rises,  the  armature  current  falls,  and  the  I R  drop  in  the 
first  motor,  A,  standing  stationary,  is  lowered,  while  B 
absorbs  a  still  greater  proportion  of  the  impressed 
E.  M.  F. 

The  manifest  remedy  for  this  state  of  affairs  is  to 
rigidly  connect  the  armatures,  so  that  when  B  starts  up 
A  is  also  set  in  motion.  A  would  then  develop  a  C.  E. 
M.  F.,  build  up  a  field,  and  take  a  share  of  the  load. 

In  the  case  of  the  series  motors  connected  in  series, 
the  case  is  modified  in  that  both  motors  start  up,  and 
attain  a  good  speed;  but  if  a  load  is  put  onto  one,  its 
speed  will  go  down,  while  that  of  the  free  motor  will  go 
up,  until  the  loaded  one  is  stopped  and  the  free  one  is 
racing.  The  reason  is  that  the  system  is  an  unstable 
one,  and  when  once  the  equilibrium  is  upset  it  does  not 


MOTOR     TESTING.  503 

return  to  a  balance,  but  the  disparity  becomes  greater 
and  greater.  Thus  when  both  are  running  the  E.  M.  F. 
may  be  said  to  be  equally  divided  between  the  two 
machines,  but  as  the  load  is  put  on  the  speed  of  the 
motor  falls,  its  C.  E.  M.  F.  falls,  and  the  potential 
difference  across  the  machine  is  lessened,  and  that  across 
the  other  machine  is  increased  so  that  its  speed  rises. 
This  rise  of  speed  in  the  free  motor  raises  its  C.  E.  M.  F., 
thus  robbing  the  loaded  motor  of  further  voltage,  so  that 
its  speed  continues  to  fall  until  at  last  it  stops.  Here 
again  the  remedy  is  to  rigidly  connect  the  armatures. 

In  experimenting  with  motors  in  series,  it  is  wise  lo- 
use only  that  voltage  which  each  can  safely  stand  singly; 
for  if  one  stops  practically  the  whole  E.  M.  F.  is  applied 
to  the  running  motor,  and  it  may  suffer  injury.  If  the 
plan  of  belting  both  motors  to  the  same  countershaft  is 
adopted,  care  must  be  taken  that  the  connections  are  such 
as  to  drive  the  shaft  in  the  same  direction.  It  is  equally 
important  that  the  pulleys  or  gear  wheels  be  of  such  size 
that  each  motor  shall  run  at  its  proper  speed.  Failure 
to  comply  with  this  requisite  may  result  in  an  overload  of 
one  motor,  and  finally  in  burning  it  out.  In  making 
these  adjustments  it  is  well  to  first  run  the  motors  alone, 
and  to  take  the  proper  observations. 

If  a  motor  designed  to  run  at  i,  200  revolutions  per  min- 
ute is  rigidly  geared  to  one  designed  for  800  revolutions  per 
minute  in  such  a  way  that  they  must  have  the  same  speed ; 
the  resultant  speed  will  be  a  compromise  between  the  two. 
As  to  which  motor  is  overloaded,  depends  upon  whether 
they  are  connected  in  series  or  in  multiple.  If  they  are  in 
series  the  current  is  the  same  in  both,  and  the  amount  of 
work  done  by  each  depends  upon  the  distribution  of  im- 


504  TESTING    OF    DYNAMOS    AND    MOTORS. 

pressed  E.  M.  F. ;  and  this  in  turn  depends  upon  the 
C.  E.  M.  Fs.,  and  in  lesser  degree  upon  the  ohmic  resist- 
ance. Any  properly  designed  motor  will  for  any  current 
generate  its  proper  C.  E.  M.  F.  when  running  most  nearly 
to  its  rated  speed,  hence  that  motor  which  is  nearest  its 
proper  speed  will  have  most  nearly  its  proper  load,  and 
the  other  one  must  take  the  balance.  Thus  if  A  is 
designed  for  1,200  revolutions  per  minute  and  is  running 
at  900  revolutions  per  minute,  and  B  is  designed  for  800 
revolutions  per  minute,  then  B  has  about  its  proper  load 
and  A  may  be  overloaded. 

If  the  machines  are  in  multiple  then  that  machine 
which  has  the  highest  relative  speed  has  the  highest 
C.  E.  M.  F.,  and  the  lowest  current  in  it.  It  thus 
throws  the  load  onto  the  machine  which  is  running  below 
its  proper  speed. 


From  the  preceding  pages  and  chapters  some  con- 
ception can  be  formed  of  the  amount  of  manipulation 
and  contriving  that  must  necessarily  be  resorted  to  in 
large  testing  rooms,  whsre  machines  of  different  output 
and  various  types  are  tested ;  where  machines  of  the  same 
output  range  in  voltage  from  25  to  500  volts,  and  are  of 
such  dimensions  that  all  the  armatures  can  be  run  in  the 
same  field  frame,  if  the  proper  precautions  are  taken;  or 
where  the  field  excitation  may  be  varied,  and  the  field 
strength  on  all  made  the  same,  and  a  difference  made  in 
E.  M.  F.  or  speed.  In  bringing  the  chapter  to  a  close 
some  stray  facts  may  be  gathered  up,  and  some  of  the 
less  frequently  met  with  difficulties  mentioned. 


MOTOR     TESTING.  505 

On  fields  having  the  same  excitation  or  ampere-turns, 
the  number  of  watts  consumed  will  be  the  same  but  the 
resistance  of  500  volt  fields  will  be  twice  that  of  250 
volt  fields,  or  four  times  that  of  125  volt  fields.  Taking 
the  bipolar  machine  as  the  simplest  type,  involving  the 
use  of  but  two  spools,  we  may  say  that  a  pair  of  500  volt 
fields  placed  in  series  on  500  volts  give  the  same  excita- 
tion and  consume  the  same  amount  of  energy  as  when 
placed  in  multiple  on  250  volts. 

In  running  a  125  volt,  250  volt,  or  500  volt  armature, 
as  a  motor  or  generator,  in  any  of  the  various  fields,  care 
must  be  taken  that  they  are  properly  connected,  and 
when  necessary  the  fields  separately  excited.  One  unac- 
customed to  handling  machines  in  this  way  is  apt  to 
forget  that  it  will  not  do  to  place  a  125  volt  field  across  a 
500  volt  armature,  and  he  may  be  forcibly  reminded  of 
his  error  by  a  burnt  out  field  or  rheostat.  Where  the 
fields  are  separately  excited  care  must  be  taken  that  there 
is  no  metallic  connection  between  field  and  armature.  A 
mistake  in  this  regard  may  throw  the  machine  and  its 
exciter  together,  either  in  series  or  multiple,  with  the 
result  that  the  field  is  unduly  strengthened  in  the  first 
case,  or  weakened  in  the  second.  If  this  mistake  is  made 
on  a  separately  excited  loss  supplier  in  a  motor-generator 
test,  it  is  likely  to  manifest  itself  in  uncalled-for  variations 
of  speed. 

As  a  general  thing  it  is  only  the  shunt  fields  that  are 
ever  connected  in  multiple,  but  sometimes  occasions 
arise  when  an  armature  is  run  in  a  compound-wound  field 
whose  current  capacity  exceeds  the  capacity  of  the  series 
coils.  In  such  a  case  the  series  coils  can  be  placed  in 
multiple  to  avoid  cutting  them  out  of  service,  care  being 


5°6 


TESTING    OF    DYNAMOS    AND    MOTORS. 


taken  that  they  are  so  connected  as  to  strengthen  and 
not  weaken  the  fields. 

It  sometimes  happens  that  it  is  desired  to  reduce  the 
E.  M.  F.  of  a  dynamo  below  its  critical  value.  Separate 
excitation  facilitates  this  without  the  danger  of  the  ma- 
chine dropping  its  field;  and  this  is  often  resorted  to 
where  current  is  to  be  sent  through  a  very  low  resist- 
ance, which  is  to 
be  measured  by  the 
method  of  fall  of 
potential.  This  ex- 
treme weakening  of 
the  field  requires 
the  introduction  of 
a  great  deal  of  re- 
sistance in  the  field 
circuit,  and  a  suffi- 
cient number  of 
rheostats  is  not 


QD-WWWH 


FIG.  163. 


always  available.  A  very  neat  way  of  avoiding  this  diffi- 
culty is  to  connect  half  of  the  field  spools  in  opposition  to 
the  other  half,  and  to  place  the  two  halves  in  multiple.  A 
resistance  box  is  then  placed  in  series  with  one  half  as 
shown  in  Fig.  163.  As  long  as  the  box  is  cut  out  and 
the  current  in  the  two  sides  is  the  same,  the  fields  oppose 
each  other  and  the  armature  voltage  is  zero.  If  now  Jt 
is  gradually  cut  in,  the  opposing  force  of  F  is  weakened 
and  ^  produces  a  field,  and  the  armature  a  correspond- 
ing E.  M.  F.  This  method  of  regulation  is  becoming 
much  used  in  putting  a  full  load  current  on  a  machine  at 
a  very  low  voltage,  for  the  purpose  of  a  heat  test.  In 
this  way  a  dynamo  may  be  short  circuited  through  an 


MOTOR     TESTING.  507 

ammeter  and  full  current  gotten  with  an  E.  M.  F.  of 
three  or  four  volts.  In  the  case  of  the  large  Edison 
multipolar  direct  driven  machines,  the  armatures  are 
always  tested  in  this  way,  without  the  machine  being 
assembled  in  the  factory. 

The  same  range  that  is  secured  for  the  E.  M.  F.  of  the 
dynamo  by  separate  excitation  is  secured  for  the  speed  of 
the  motor  by  the  same  means.  In  this  case  the  fields  are 
connected  in  multiple,  and  by  raising  the  voltage  the 
field  is  made  very  strong  and  the  speed  correspondingly 
reduced. 

This  method  of  field  excitation  probably  has  its  ex- 
treme example  in  the  following  method,  which  has  given 
satisfaction  in  the  hands  of  the  writers,  and  was  called 
into  use  when  the  field  rheostats  were  not  of  sufficient 
current  capacity  to  be  placed  in  the  field  circuit  whose 
resistance  was  to  be  varied.  The  method  consists  in  ex- 
citing the  fields  from  two  generators  connected  in  oppo- 
sition. When  the  voltage  of  the  generators  is  equal  and 
opposite  no  current  flows  and  there  is  no  zero  field.  By 
lowering  the  voltage  on  one  machine  a  current  flows  of 
the  value 

E  -  E' 


E  and  E  being  the  E.  M.  F.  of  the  two  generators  and 
r  the  resistance  of  the  field  circuit.  To  get  the  best 
results  the  two  generators  should  be  themselves  sepa- 
rately excited. 

On  separately  excited  machines  the  brushes  seem  to 
spark  less  for  the  same  load  variation  than  do  those  that 
are  self-excited;  also  the  variation  of  potential  attheter- 


508  TESTING    OF    DYNAMOS    AND    MOTORS. 

minals,  for  a  given  change  in  the  current,  is  less  on  a 
separately  excited  than  on  a  self-excited  shunt  dynamo. 
The  reason  for  this  is  that  any  change  in  the  armature 
current  produces  a  weakening  of  the  fields,  while  on  a 
separately  excited  machine  the  fields  are  constant  aside 
from  the  slight  effect  of  armature  reaction. 

If,  in  separately  exciting,  the  fields  are  placed  in  multi- 
ple, it  must  be  remembered  that  the  current  carrying 
capacity  of  the  boxes  must  be  doubled,  or  a  burn  out 
may  ensue.  When  a  box  shows  signs  of  giving  out,  it 
should  be  relieved  by  placing  a  second  one  in  multiple 
with  it,  and  then  the  defective  box  cut  out.  To  cut  out 
the  box  it  is  not  always  necessary  to  remove  the  connect- 
ing wires,  but  it  is  sufficient  to  turn  the  handle  to  the 
"all  out"  position.  Unfortunately,  makers  of  rheostats 
have  not  yet  agreed  upon  a  uniform  construction  for 
rheostats,  so  that  some  turn  from  right  to  left  and  some 
from  left  to  right  to  accomplish  the  same  result.  Thus 
to  introduce  resistance,  the  handle  of  an  Edison  box  is 
turned  from  left  to  right,  while  with  the  Thomson-Hous- 
ton box  the  opposite  is  true.  These  minor  points  must 
be  familiar  to  the  tester,  for  a  wrong  movement  at  a 
critical  time  may  precipitate  serious  trouble. 

A  recent  writer  has  drawn  attention  to  the  need  of  uni- 
formity in  the  rating  of  and  specifications  for  dynamos 
and  motors.  Ample  justification  can  be  found  in  the 
preceding  chapters  for  this  remark,  and  in  closing  this 
survey  of  electrical  testing,  the  writers  would  add  their 
voice  to  this  timely  plea.  No  tester  who  has  been  long 
upon  the  floor  but  knows  the  added  troubles  resulting 
from  even  slight  differences  of  detail  in  the  instructions 
for  the  various  tests.  Perhaps  no  one  thing  would 


MOTOR     TESTING.  509 

more  help  to  make  electrical  testing  Simpler,  and  freer 
from  needless  annoyances,  than  such  a  reform.  Ex- 
perience would  soon  crystallize  into  better-defined  and 
more  uniform  methods,  and  the  special  test  no  longer 
arise,  like  a  spectre,  to  trouble  the  dreams  of  the  Weary 
Tester. 


CHAPTER  XV. 

INSTALLATION   CAR   EQUIPMENT  TESTS. 

The  subject  of  testing  a  railway  car  equipment  will, 
in  this  chapter,  be  considered  from  the  standpoint  of  the 
operator,  rather  than  from  that  of  the  designing  or  in- 
stalling engineer.  It  will  be  assumed  that  the  equipment 
—motors,  controllers,  trucks,  etc. — has  been  purchased 
from  a  responsible  concern  and  is  in  every  way  up-to- 
date  in  design  and  workmanship.  It  is  quite  safe  under 
present  conditions  of  competition  to  assume  that  all  ques- 
tions of  efficiency,  temperature  rise,  speed,  torque  and 
current  consumption  have  been  reduced  to  such  a  basis, 
as  to  be  almost  as  readily  comprehended  by  the  buyer  as 
by  the  maker. 

The  controller  maker,  for  example,  has  so  completely 
mastered  all  the  points  involved  in  starting  and  stopping 
a  car  under  various  conditions  of  load,  grade,  speed, 
schedule  and  even  handling,  that  he  knows  just  about 
what  device  to  recommend  to  fulfil  certain  maximum 
conditions  of  use  and  abuse  in  a  given  service.  So  also, 
the  truck  builder  is  willing  to  vouch  for  his  part  of  the 
equipment,  feeling  that  in  the  article  offered,  a  liberal 
factor  of  safety  based  upon  experience  makes  all  due  al- 
lowance for  square  curves,  bad  rails,  and  to  some  extent, 
lack  of  attention  to  wearing  parts.  The  claims  of  the 

510 


INSTALLATION   CAR   EQUIPMENT  TESTS.  51 1 

various  parties  relative  to  the  excellence  of  their  wares 
can,  as  a  rule,  be  conceded,  provided  they  show  a  willing- 
ness to  waive  payment  until  a  fair  trial  in  service  shall 
confirm  their  claims. 

After  a  high  grade  equipment  has  been  secured,  in- 
stalled and  is  in  satisfactory  operation,  eternal  vigilance 
is  the  price  of  continued  good  service  and  the  object 
here  will  be  to  point  out  the  conditions  of  abuse 
under  which  trouble  is  most  apt  to  arise  in  the  several 
parts  of  the  equipment,  to  show  how  to  avoid  these  condi- 
tions and  to  give  the  most  common  methods  of  locating 
and  remedying  the  faults  to  which  even  the  best  of 
equipments  must  ever  be  liable.  There  is,  perhaps,  no 
more  logical  order  in  which  to  discuss  the  devices  com- 
posing the  equipment  of  a  car  than  to  take  them  up  in 
the  order  in  which  the  current  passes  through  them  on 
its  way  from  the  trolley  wire  to  the  rail.  This  order  will 
be  as  follows  :  I  The  trolley,  II  The  Overhead  Switches, 
III  Fuse  Box,  IV  Lightning  Arrester,  V  Controller  (in- 
cluding the  starting  coil  and  motors.) 

I  THE  TROLLEY.  This  device  comprises  two  prin- 
cipal parts — the  stand  and  the/WI?.  The  stand  comprises 
an  upper  pivotal  member  called  the  base  and  a  lower 
stationary  member  called  the  foot.  Fig.  164  shows  a 
base  complete:  C  is  the  foot,  all  the  rest  of  the  device  is 
the  base.  C  screws  to  the  top  of  the  car  and  acts  as  a 
pivot  for  the  base  to  turn  on.  A  A'  is  a  single  casting, 
the  upper  end  of  which  is  a  socket  adjustable  to  receive 
the  pole;  the  lower  end,  A ',  is  forked  or  yoke  shaped 
and  engages  the  compression  rods,  Z>,  that  actuate  spring 
F,  when  the  pole  is  pulled  down.  A  A'  turns  on  center 

• 


512 


TESTING   OF   DYNAMOS   AND   MOTORS. 


f.  Stem  G  is  fixed  to  casting  B  B,  and  serves  to  sup- 
port and  guide  compression  cups,  IT,  and  the  springs 
F  and  E,  Spring  E  is  the  kick  spring  and  H  is  a  nut 
used  to  vary  the  compression  on  F.  The  action  of  the 
stand  is  as  follows:  in  the  figure  it  is  in  its  neutral  posi- 
tion and  the  pole  should  stand  nearly  vertical.  To  put 
the  pole  on  the  wire  it  must  be  rocked  down  clock- 
wise; this  swings  A'  clockwise  around  J,  moves  rods 


FIG.  164. 

D  to  the  left,  and  with  them  compression  cup,  /', 
thereby  compressing  F.  The  lower  the  pole  is  pulled, 
the  more  is  F  compressed  and  the  more  upward  pressure 
does  the  pole  exert  against  the  wire.  Spring  E  is  pro- 
vided to  cushion  the  blow  received  should  the  pole  get 
away  in  service.  Suppose  the  rope  breaks;  as  soon  as 
A  A'  rocks  past  the  neutral  position,  rods,  D  D,  pull 
on  r  and  compress  E\  this  cushions  the  blow  and  lessens 
the  chances  of  breaking  any  of  the  parts.  By  tightening 
nuts  If,  cup  /  is  forced  along  stem  G,  compressing  spring 


INSTALLATION   CAR   EQUIPMENT  TESTS.  513 

/',  and  thereby  increasing  the  upward  pressure  of  the 
pole  at  all  positions.  To  keep  a  stand  in  good  W0rking 
order,  pins  J  and  K  and  the  pivot  around  which  base  B  H 
turns  on  C  C,  should  be  oiled  auout  once  a  week. 
Also  alxMit  once  a  month,  stiff  grease  should  be  applied 
to  stem  G  so  spring  F  may  work  freely.  If  G  and  J  are 
not  kept  lubricated,  the  action  of  the  whole  device  be- 
comes sluggish,  the  pole  will  not  work  up  and  down 
freely  and  will  tend  to  jump  the  wire  even  on  a  straight 
track.  If  the  main  pivot  is  allowed  to  run  dry,  the  pole 
is  sure  to  leave  the  wire  on  curves  and  cross-overs.  The 
first  symptom  that  a  trolley  stand  needs  oil,  is  that  the 
whole  thing  works  hard  when  the  wheel  is  placed  on 
the  wire  from  which  it  has  jumped.  With  neglect  all 
moving  parts  wear  loose. 

THE  POLE.  This  comprises  the  pole  proper,  the  ferrule 
the  harp  and  the  wheel.  Fig.  165  shows  a  pole  complete; 
P,  is  the  pole  proper;  H,  the  harp;  F.  the  ferrule;  St  the 
contact  spring  and  W,  the  wheel.  Most  poles  are  made 
of  hard  drawn  steel,  which  will  bend  only  under  a  heavy 
force;  once  bent  such  a  pole  should  be  straightened  cold 
as  the  character  of  the  steel  is  such  that  it  looses  temper 
if  heated.  Poles  are  from  12  to  16  ft.  long,  according  to 
the  height  of  the  car  and  the  height  of  the  wire.  1 1  is 
i^  in.  diameter  on  the  big  end,  holds  this  diameter  for 
about  2  ft.  and  then  tapers  to  i  in.  on  the  harp  end.  The 
ferrule  is  a  brass  or  iron  ring  with  an  eye  hole  to  take 
the  trolley  rope.  It  is  free  to  turn  on  the  harp  stem  as  a 
center.  The  harp,  H,  is  a  brass  or  malleable  iron  fork 
designed  to  be  riveted  on  to  the  small  end  of  the  pole  to 
hold  the  wheel,  If.  Iron  has  almost  entirely  supplanted 


TESTING  OP  DYNAMOS  AND  MOTORS. 


brass  in  such  uses,  as  it  offers  less  temptation  to  thieves 
and  costs  less.     The  tangs  of  the  fork  are  drilled  to  take 
the  axle,  A,  around  which  W  turns  as  a  center.     The 
axle  is  secured  on  both  ends  by  a  cotter 
pin,  which  lies  in  a  groove  and  keeps  the 
axle  from  turning.     A  good  harp  saves 
line  work,  the  main  requirement  being 
that  it  have  110  corners  or  swells  to  catch 
in  a  line  frog  if  the  pole  goes  off  under 
headway.      The  harp   should   be   light. 
The  wheel  is  made  of  brass  or  gun  metal ; 
if  too  soft  its  life  is  short,  -if  too  hard  it 
wears   the  wire.     The  amount  of  wear 
between  the  two  also  depends  upon  the 
shape  of  the  groove,  the  condition  of  the 
line  work  and  the  care  bestowed   upon 
the  adjustment   of   the   base  and   pole. 
Some  wheels  are  solid,  others  have  re- 
movable   centers    called    bushings.       A 
trolley  bushing  is  a  spiral  brass  casting 
whose  outside  is  hollow  milled  to  drive 
into  a  hole  in  the  wheel  and  whose  inside 
is  bored  to  fit  the  axle.     The  air  chan- 
nels beeween  the  metal  spirals  are  filled 
with  a  graphite  compound.      As  the  bear- 
FIG.  165.         ing  is  the  first  part  to  wear  out  under 
normal  conditions,  the   use   of   a   bush- 
ing prolongs  the  life  of   a  wheel.     To  replace  a   worn 
bushing,  it  is  driven  out  and  a  new  one  driven  in.    L,ives 
of  trolley  wheels  vary  widely  with  local  conditions,  but 
should  average  5000  miles  under  good  care. 


INSTALLATION    CAR    EQUIPMENT   TESTS.  51 5 

Every  road  should  experimentally  determine  the  trolley 
characteristics  best  suited  to  its  own  conditions.  By 
trolley  characteristics  are  meant :  pole  tension,  weight 
and  material  of  the  pole  ;  size,  weight,  shape  and  mate- 
rial of  the  wheel  ;  width,  depth  and  general  contour  of 
the  groove  ;  shape  of  the  groove  lips  or  flanges.  Such 
data  can  be  gotton  only  by  a  series  of  careful  tests.  Such 
a  series  of  tests  alwrays  discloses  facts  not  looked  for  and 
should  be  conducted  with  great  care  so  that  results  may 
be  relied  upon.  The  test  consists  in  installing  several 
kinds  of  wheels  in  service  and  keeping  track  of  their 
records.  Each  kind  should  be  put  on  at  least  six  cars 
and  on  more  if  possible.  The  mileage  of  each  wheel 
must  be  kept  exactly,  noting  delays,  accidents,  etc.,  and 
one  wheel  of  each  kind  must  be  kept  so  that  the  shapes 
of  the  grooves  of  the  old  wheels  and  new  ones  can  be 
compared.  As  the  condition  of  the  trolley  stand  influ- 
ences the  life  of  the  wheel  greatly,  a  series  of  preliminary 
tests  must  be  made  to  insure  that  all  of  the  wheels  start 
out  on  an  even  basis.  All  poles  must  be  of  the  same 
size,  weight  and  length  and  must  be  of  the  same  tension. 
The  tester  must  be  certain  that  all  parts  of  the  trolley 
outfit  are  greased  and  kept  greased,  so  that  the  motion 
of  the  wheel  on  the  axle,  of  the  socket  up  and  down  and 
cf  the  base  on  the  foot  pivot  will  always  be  free  and  easy. 
When  a  trolley  outfit  is  set  up  correctly,  all  parts  are  well 
oiled  and  aligned,  the  harp  is  on  the  pole  straight  and 
the  pole  is  in  the  socket  straight,  so  that  the  wheel  when 
on  a  straight  wire  has  its  flanges  parallel  to  the  wire. 
To  insure  that  all  these  conditions  obtain,  select  inside  of 
the  shed,  a  stretch  of  the  wire  about  50  ft.  long,  drop  a 


516  TESTING  OF  DYNAMOS  AND  MOTORS. 

plumb  line  down  from  the  ends  to  the  track  below  and  draw 
a  chalk  line  through  the  two  points  touched  by  the  bob. 
This  chalk  line  should  be  everywhere  equidistant  from 
the  two  rails ;  if  it  is  not  so,  the  trolley  wire  must  be 
pulled  over  until  it  is.  The  car  can  now  be  run  under 
the  truly  centered  wire  and  the  trolley  device  adjusted 
to  the  proper  tension  (about  16  Ibs.  as  a  rule)  and  all  the 
parts  aligned.  If  the  adjustment  is  good,  the  pole,  when 
on  the  wire,  will  lean  to  neither  one  side  or  the  other, 
the  wire  will  rest  in  the  bottom  of  the  groove  and  will  be 
parallel  to  the  flanges.  The  set  should  be  the  same  from 
both  ends  of  the  car,  and  in  the  case  of  double  spring 
bases  whose  poles  rock  over,  as  well  as  swing  around, 
the  tension  should  be  the  same  in  both  cases.  Rock-over 
bases  have  the  advantage  that  if  the  pole  gets  bent  con- 
ditions can  be  improved  by  rocking  it  over  so  as  to  bring 
the  bend  down.  Sometimes,  even  when  the  parts  are 
all  well  oiled  and  adjusted,  the  base  will  work  hard,  be- 
cause it  is  loose  on  the  pivot,  due  to  wear,  and  binds 
when  one  tries  to  swing  it.  This  fault,  as  a  rule,  occurs 
only  on  old  stands. 

Assuming  the  bases  and  poles  to  be  set  right,  the  fol- 
lowing conclusions  can  be  drawn  in  regard  to  the  wear  of 
the  wheels.  If  both  flanges  of  the  wheels  persist  in 
wearing  to  a  razor  edge  before  the  bushing  is  much  worn, 
the  groove  is  too  deep  or  too  narrow,  or  both,  or  the  in- 
side edge  of  the  flange  is  not  flared  enough.  If  the  wire 
wears  down  into  one  side  of  the  groove  bottom  and  one 
flange  only  gets  sharp,  the  trolley  wire  is  out  of  center 
somewhere  or  the  pole  is  out  of  set.  The  first  conclusion 
is  verified  if  the  same  wheel  does  not  give  this  trouble  on 


INSTALLATION   CAR   EQUIPMENT   TESTS.  517 

other  parts  of  the  road.  Where  the  wire  cuts  down  into 
the  center  of  the  groove  very  rapidly,  it  prooves  that  the 
metal  is  too  soft  and  the  tension  too  strong  for  that  par- 
ticular make  of  wheel.  A  chattering  noise  when  in 
motion  indicates  either  that  the  wheel  is  flat  or  the  bush- 
ing worn.  A  flat  wheel  may  be  due  to  lack  of  oil  causing 
it  to  stick,  or  to  a  bent  harp  or  to  soft  spots  in  the  metal. 
A  worn  bushing  may  be  due  to  want  of  oil  or  to  soft- 
ness. The  makers  may  get  it  too  soft  in  the  effort  to 
have  it  self-lubricating.  It  is  better  to  have  it  hard 
enough  and  to  keep  it  oiled.  Also  the  axles  themselves 
wear  small  in  course  of  time  ;  all  wheels  should  be  gauged 
as  well  as  the  axles.  Occasionally  a  road  will  get  hold 
of  a  lot  of  wheels  that  are  bored  out  of  center  and  that 
will  cause  them  to  emit  the  flat  wheel  chatter.  Where 
the  wheels  show  a  rough  pitted  appearance  on  the  lip  of 
the  flanges,  it  shows  poor  handling  in  running  through 
overhead  frogs  where  the  flange  must  carry  all  the  cur- 
rent ;  the  same  roughness  is  carried  to  a  degree  where  the 
poor  design  of  the  line  causes  the  wheel  to  jump  at  every 
ear.  The  rougher  a  wheel  is,  the  more  apt  is  it  to  leave 
the  wire  at  curves  and  crossings.  Finally  the  best  shape 
for  a  wheel  is  indicated  by  the  wear  of  those  that  have 
been  in  service. 

THE  SPRING.  The  trolley  wheel  spring  is  made  of 
copper  and  serves  two  purposes  ;  it  acts  as  a  soft  metal 
washer  between  the  hub  of  the  wheel  and  the  side  .of  the 
harp  and  saves  wear.  It  also  conducts  the  current  from 
the  wheel  to  the  harp.  Where  no  spring  is  used,  the  bear- 
ing surface  between  the  wheel  and  axle  must  carry  most 
of  the  current  with  the  result  that  i*  gets  pitted  and  rough. 


TESTING  OF  DYNAMOS  AND  MOTORS. 


II  THE  OVERHEAD  SWITCH.  This  device  is  placed 
under  the  hood  just  above  and  a  little  in  front  of  the 
motorman's  head,  with  its  handle  so  turned  that  the 
most  involuntary  movement  of  the  motorman  in  the  time 
of  trouble  will  be  to  slap  it  "  Off."  The  object  of  the 


G — 


FIG.  1 66. 


device  is  to  provide  a  simple  and  certain  means  of  break- 
ing the  current,  should  a  ground  or  other  fault  render 
the  controller  unable  to  do  so.  When  this  switch  is  open, 
no  current  can  get  to  the  motors  or  controllers  and  so  it  is 
used  for  ' '  killing ' '  the  circuit  wrhen  it  is  desired  to  in- 
spect or  work  on  any  part  of  it.  Again,  there  may  be 


INSTALLATION    CAR    EQUIPMENT   TESTS.  519 

trouble  with  the  controller  or  some  other  device,  such 
that  it  may  be  convenient  to  put  the  controller  on  the  ist 
notch,  and  run  the  car  to  the  house  by  means  of  this  over- 
head switch.  The  most  complete  type  of  the  device  in 
use  to-day,  is  the  combined  overhead  switch  and  cir- 
cuit breaker.  Fig.  166  shows  such  a  device  very  much 
in  use.  In  the  Fig.  A,  is  the  positive  terminal;  /?,  the 
negative;  C,  a  coil  which  operates  the  tripping  device  and 
also  operates  the  blow-out;  2),  the  push  button  by  means 
of  which  the  tripping  device  may  l)e  operated  by  hand;  /?, 
a  spring  that  holds  the  keeper,  A',  away  from  its  coil,  C\ 
unless  the  current  exceeds  the  value 
at  which  the  breaker  is  set  to  act  ;  JL.  JL 

F  is  a  graduated  scale  ;  /  is  the 
sighting  disc,  by  means  of  which 
and  thumbscrew,  G,  the  tension  on 
E  is  regulated;  H,  is  the  handle  C1 
for  resetting  the  breaker.  Fig. 
167  is  a  diagramatic  sketch  of  the 
connections  revealed  by  removing 
the  name  plate.  A,  B,  C  and 
//  are  the  same  as  in  Fig.  166.  The 
breaker  is  as  follows  :  suppose  G  is  adjusted  till  /  is  on  a 
level  with  175  on  the  scale;  the  breaker  is  then  supposed 
to  act  at  175  amps.  For  any  current  less  than  this,  E 
holds  K  against  the  pull  of  C  ;  at  175  amps,  the  pull  of 
C  overcomes  E  and  draws  A' down,  thereby  liberating  the 
trigger  on  K  from  a  shoulder  carried  on  handle,  H.  A 
spring  then  carries  the  handle  to  the  "off"  position, 
opening  the  circuit  between  E  and  E.  The  arc  formed 
moves  up  to  the  auxiliary  breaking  blocks  E ',  17,  and  is 


520  TESTING   OF   DYNAMOS   AND   MOTORS. 

there  extinguished  by  the  magnetic  action  also  provided 
by  coil  C.  For  use  on  cars  the  breaker  is  enclosed  in  a 
wooden  case,  out  of  which  sticks  the  handle,  A,  and  the 
push  button,  D.  To  reset  the  device  after  action,  the 
handle  is  shoved  to,  the  same  as  on  any  ordinary  switch. 
To  operate  the  switch  by  hand,  simply  press  the  button; 
this  shuts  the  keeper  down  the  same  as  the  current  does 
on  overload  and  liberates  the  handle. 


FIG.  168. 

There  are  several  important  points  to  be  watched 
about  a  circuit  breaker,  ist.  It  must  be  tested  at  inter- 
vals to  see  that  it  acts  at  the  current  value  for  which  it 
is  set.  To  do  this  have  a  reliable  ammeter  and  a  water 
rheostat  connected  in  series,  as  indicated  in  Fig.  168, 
where  A  is  the  meter;  R,  the  water  rheostat  and  K  an 
ordinary  switch.  Test  line  T  terminates  in  a  hook  that 
can  be  hooked  over  the  negative  post  on  the  breaker, 
and  all  disconnecting  being  thereby  avoided.  To  test  a 
breaker  on  a  car,  throw  the  controller  off  and  hook  test  line 


INSTALLATION   CAR    EQUIPMENT   TESTS. 


521 


T  on  to  the  negative  side  of  the  breaker;  after  removing 
the  cover,  note  the  value  to  which  the  breaker  is  set;  if  the 
value  is  right,  close  K  and  work  the  current  up  to  that 
value  ;  the  breaker  should  act  within  5  amps,  of  it. 


TROLLEY 


FIG.  169. 

If  it  fails  to,  readjust  by  means  of  G.  2nd.  The  blow, 
out  chamber  should  not  be  allowed  to  accumulate  carbon 
dust.  At  least  once  a  year  the  insulation  between  the 
break  points  should  be  measured  as  follows  :  as  indicated 
in  Fig.  169,  run  a  test  line  from  the  trolley  wire  to  the 
positive  side  of  a  500  volt  voltmeter;  from  the  negative  side 
of  the  meter  run  a  second  test  line  terminating  in  a  hard 
point ;  next  tie  down  the  trolley  pole  and  see  that  the 
breaker  is  opened,  and  put  one  controller  on  the  first  notch. 
The  test  consists  in  touching  the  positive  side  of  the  open 


522  TESTING  OF  DYNAMOS  AND  MOTORS. 

breaker  with  the  free  test  line.  If  the  arc  chamber  is  car- 
bonized, a  current  flows  from  the  trolley  wire,  through  the 
voltmeter,  across  the  carbonized  path  to  the  ground.  If 
the  deflection  is  more  than  450  volts,  the  name  plate  should 
be  removed  and  the  arc  chamber  cleaned.  3rd.  Under  no 
circumstances  should  the  breaker  be  placed  where  the 
driver  cannot  reach  the  handle  without  jumping  ;  if  it  is,  he 
will  use  a  switch  iron  with  the  result  that  in  a  short  while 
the  interference  lug  on  the  handle  will  be  so  impaired 
that  the  breaker  cannot  be  set  ;  if  this  occurs  on  the  road 
it  will  be  necessary  to  cut  the  device  out  entirely  or  to  tie 
the  handle  over  in  order  to  run  the  car  to  the  house. 
4th.  Where  a  breaker  has  been  neglected  and  allowed  to 
stick,  or  where  the  capacity  of  the  motors  has  been  in- 
creased without  regard  to  the  breaker,  the  insulation  on 
the  wire  of  the  blow  coil  may  get  so  badly  roasted  as  to 
short  circuit  it.  In  such  a  case,  the  coil  will  not  operate 
at  the  value  set  and  hence  the  device  is  of  little  pro- 
tection to  the  motors  ;  also,  when  it  does  act,  there  is  bad 
arcing  in  the  arc  chamber.  When  there  is  reason  to 
suppose  that  the  blow  coil  is  baked,  it  can  of  course  be 
tested  by  one  of  the  several  ways  given  in  Chaper  VI, 
but  the  best  way  is  to  remove  a  part  of  the  insulation 
from  the  coil  and  look  at  the  wire  ;  if  the  cover  is  brown 
and  can  be  scraped  off  with  the  nail,  it  is  baked,  and  the 
coil  must  be  renewed.  When  a  coil  is  baked,  there  is 
always  a  protracted  characteristic  sputtering  when  the 
arc  is  extinguished.  When  the  breaker  is  in  good 
order,  there  is  no  noise  save  a  puif  somewhat  similar  to 
the  exhaust  of  a  loaded  engine. 


INSTALLATION   CAR    EQUIPMENT   TESTS.  523 

The  strongest  point  about  a  breaker-overhead-switch  is 
that  it  will  not  allow  a  motorman  to  "notch"  his  controller 
above  a  certain  rate,  and  in  emergency  reversals  he  does 
not  put  the  controller  beyond  the  first  notch.  Unscrupu- 
lous men  can  and  do  get  around  this  feature  sometimes  by 
holding  the  handle  over  with  a  switch  iron,  but  such  prac- 
tice should  be  met  by  severe  discipline.  A  car  breaker  is 
much  more  of  a  protection  than  a  car  fuse,  as  is  evidenced 
by  the  fact  that  cars  run  into  the  house  on  account  of 
roasted  fields,  could  not,  after  being  equipped  with  a 
breaker  set  to  act  at  the  fuse  rating,  be  even  started,  be- 
cause the  breaker  would  act  on  the  first  notch. 

On  all  up-to-date  cars  there  is  a  hood-switch  or  breaker 
on  both  ends,  so  that  in  time  of  trouble  the  cutting  off  of 
the  power  is  under  the  control  of  both  members  of  the 
crew.  It  is  safe  enough,  and  less  expensive,  to  have  a 
plain  switch  on  one  end,  and  a  breaker  on  the  other. 

Ill  THE  FrsE-Box. — This  is  also  a  safety  device.  As  a 
rule  a  car  has  a  fuse-l>ox,even  if  it  has  a  breaker,  and  where 
there  is  no  breaker  it  is  absolutely  essential  that  there  be  a 
fuse  to  protect  the  devices,  in  case  of  a  short  circuit.  The 
weakest  part  of  a  string  will  break ;  so,  also,  the  weakest 
part  of  an  electric  circuit  will  burn  out  when  the  circuit 
is  overstrained.  This  weakest  part  might  prove  to  be  a 
loose  connection  or  a  bad  contact,  but  it  is  more  than  likely 
that  the  weak  spot  will  show  up  inside  of  a  motor  where  it 
costs  more  to  repair  the  damage.  The  idea  of  the  fuse-box 
is  to  provide  a  weak  spot,  which,  in  case  of  a  short  circuit, 
will  give  way  before  any  other  spot  does.  To  insure  this, 
the  fuse,  if  of  copper,  is  made  smaller  than  any  other  wire 
to  be  found  in  the  main  motor  circuit  or  any  of  the  other 


524  TESTING  OF  DYNAMOS  AND  MOTORS. 

devices  that  carry  the  main  current.  A  30  h-p  street  rail- 
way motor  armature  is  wound  with  a  No.  8  or  No.  9  B.  & 
S.  wire,  and  the  field  with  a  No.  4  or  No.  5  B.  &  S.,  ac- 
cording to  the  work  to  be  done.  The  size  of  the  armature 
wire  is  one-half  that  of  the  field,  because  the  two  halves  of 
the  armature  are  in  multiple,  and  each  wire  carries 
but  half  the  current,  so  the  field  and  armature  have 
the  same  current  capacity.  The  fuse  wire  must,  however, 
be  a  great  deal  smaller  than  the  field  wire,  because,  being 
in  an  exposed  place,  its  facilities  for  radiation  are  greater, 
and  a  much  larger  current  is  required  to  melt  it  than  is 
required  inside  of  a  hot,  closed  motor.  Again,  the  shorter 
a  fuse,  the  smaller  must  it  be  to  melt  at  a  given  current 
value,  because  the  lugs  to  which  it  attaches  conduct  the 
heat  off.  As  a  final  result  of  all  these  influences,  it  is 
found  that  two  30  h-p  motors  should  be  protected  by  a 
fuse  wire  no  larger  than  a  No.  12  B.  &  S.,  and  two  50  h-p 
or  60  h-p  motors,  properly  handled,  can  get  along  on  a 
No.  9  B.  &  S.  This  assumes  a  distance  of  4  in.  between 
lugs. 

Every  road  using  copper  fuses  should  determine,  by 
test,  the  size  of  wire  needed  to  protect  each  type  of  equip- 
ment. Such  a  test  must  be  based  upon  a  knowledge  of 
the  full  load  capacity  of  the  motors;  their  average  daily 
current,  and  the  length  of  the  fuse  to  be  used.  The  cur- 
rent capacity  of  a  motor  is  gotten  by  multiplying  the  rated 
horse-power  by  746,  the  watts  per  horse-power,  and  divid- 
ing by  the  line  voltage.  For  example,  a  3O-hp  motor  on  a 
line  averaging  500  volts  would  have  a  current  capacity  of 

30X746  =^>3§£=  44.7  Now,  it  is  the  current 

500  500  * ' 


INSTALLATION   CAR    EQUIPMENT   TESTS.  525 

that  burns  out  a  wire,  so  it  is  not  hard  to  sec  that  where 
the  voltage  on  a  line  is  allowed  to  get  low  and  the  time 
table  is  kept  the  same,  the  motorman,  in  his  effort  to  keep 
on  time,  must  abuse  his  motors,  increase  their  average 
current  consumption,  and,  therefore,  the  size  of  the  fuse 
required.  The  average  current  consumption  can  be  got- 
ten best  by  the  use  of  a  wattmeter.  The  wattmeter  is 
connected  on  the  car  (the  field  in  series  with  the  overhead 
switch,  the  armature  across  the  line),  and  its  reading 


FIG.  170. 

taken  just  before  the  car  goes  out  on  its  first  run.  When 
the  car  comes  into  the  house  at  night  the  meter  is  read 
again ;  the  difference  between  the  two  readings  gives  the 
number  of  watt-hours  absorbed  during  the  day  of,  say, 
twelve  hours.  Dividing  this  by  1 2  and  the  voltage  on  the 
line  gives  the  average  flow  of  current  during  that  time.  In 
the  absence  of  a  wattmeter,  an  ammeter  must  be  used,  but 
this  is  very  tedious,  for,  in  order  to  get  correct  results,  the 
readings  must  follow  each  other  at  very  close  intervals,  say, 
ten  to  the  minute.  At  the  end  of  the  test  the  current  read- 
ings are  all  added  together  and  divided  by  the  number  of 
readings;  this  gives  the  average  current.  Knowing  the 


526  TESTING  "OF  DYNAMOS  AND  MOTORS. 

average  current  and  the  full  load  current,  a  wire  is  selected 
which,  at  the  given  distance  between  lugs,  will  stand  con- 
tinuously that  current  which  is  half-way  between  the  full 
load  and  average  values.  Say  the  full  load  current  of  the 
car  is  90  amps,  and  the  average  current  40  amps. ,  then 
the  current  for  which  the  fuse  must  be  selected  is  65  amps. 

Let  us  assume  that  the  length  of  the  fuse  is  to  be 
4  ins.  Two  metal  blocks  provided  with  connecting  posts 
must  be  rigged  up  as  shown  in  Fig.  170,  where  T  is  the 
trolley  wire ;  K,  a  switch ;  BB,  the  two  blocks  ;  F ,  the  fuse 
wire  R,<  a  water  resistance;  A,  an  ammeter,  and  G,  the 
ground.  //  are  two  pieces  of  asbestos  or  fiber,  through 
which  the  fuse  is  run  to  prevent  the  blocks,  BB,  from  be- 
ing burned  by  the  arc.  With  the  fuse  short-circuited,  J? 
is  adjusted  until  A  registers  65.  The  short  circuit  is  then 
removed  and  the  fuse  let  into  circuit  and  left  there  for 
about  five  minutes,  when  it  is  just  as  hot  as  it  will  ever  be. 
A  dark  box  is  then  set  over  the  fuse,  blocks  and  all,  the 
box  having  in  the  top  a  small  hole  through  which  the  fuse 
can  be  seen.  The  fuse  seen  in  the  dark  should  show  the 
faintest  pink  color ;  if  it  does  not,  a  smaller  size  wire  must 
be  selected,  and  so  on  until  the  right  size  is  obtained.  A 
fuse  selected  on  this  basis  will,  under  normal  service  con- 
ditions, blow  once  in  about  every  two  weeks,  from 
gradual  deterioration. 

EXTINGUISHING  THE  ARC. — Were  no  means  provided 
for  extinguishing  the  arc  that  tends  to  hold  when  a  fuse 
blows  under  heavy  current  on  a  5oo-volt  circuit,  the  fuse 
blocks  would  soon  be  burned  to  a  state  of  uselessness. 
Several  schemes  have  been  devised  for  suppressing  the 
arc.  Among  them  are :  confinement  in  an  air  tight 


INSTALLATION    CAR   EQUIPMENT   TESTS.  527 

chamber ;  the  use  of  spring  flaps  to  break  the  arc ;  the 
use  of  an  extra  long  fuse  ;  the  use  of  a  magnetic  blowout. 
The  magnetic  blowout  type  made  by  the  General  Flectric 
Company  has  successfully  withstood  the  test  of  years,  and 
will  be  selected  for  description. 

Fig.  171  shows  the  fuse-box  complete  ready  for  a  fuse, 
and  Fig.  172  is  a  diagrammatic  sketch  of  the  inside  con- 
nections. In  Fig.  171  are  holes,  through  which  the 
two  circuit  wires  go  to  connect  in  the  box  :  through  holes 
bb  a  screw-driver  can  be  put  to  tighten  the  connections, 

abe,   shown   in  Fig. . 7 

172;    cc    in    both         V t* 

fi  g  u  r  e  s  are  the 
thumb-screws  to  se- 
cure the  two  ends 
of  the  fuse  wire.  /, 
Fig.  171,  is  a  raw- 
hide cover  to  keep  pI(;  j^T 
out  the  water.  No 

substantial  lid  is  needed,  as  there  is  no  demonstration 
when  a  fuse  blows.  Fig.  172  (b)  shows  the  special  fuse 
used ;  a  special  fuse  is  not  absolutely  necessary,  as  it  is 
very  easy  to  secure  a  plain  wire  under  the  thumb-screws, 
but  it  is  a  good  idea,  in  that  it  %lessens  the  chances  of  the 
wrong  size  of  fuse  being  used. 

POINTS  ON  FUSE- BOXES. — On  many  roads  the  proper 
' '  fusing  ' '  of  the  cars  seems  to  be  treated  as  a  matter  of 
minor  importance.  Trucks,  motors,  etc. ,  are  shifted  from 
one  car  to  another  without  any  apparent  attention  being 
paid  to  the  requirements  of  the  fuse-box,  which  being 
fastened  to  the  car  body,  remains  there,  a  victim  of  circum- 


528 


TESTING  OF  DYNAMOS  AND  MOTORS. 


stances.  Most  roads  operate  several  capacities  of  motors  ; 
as  a  rule,  a  motorman  carries  a  bunch  of  fuses,  any  one  of 
which,  in  the  majority  of  cases,  he  regards  as  adapted  to 
use  on  any  car  he  may  happen  to  be  running.  If  allowed 
to  choose  his  own  fuse  he  is  apt  to  choose  the  biggest.  In 
this  way,  large  motors  get  small  fuses  and  small  motors 
large  fuses.  The  use  of  a  fuse  that  is  too  small  entails  no 
bad  results  beyond  the  annoyance  of  having  it  blow  con- 
tinually for  no  other  reason  than  that  it  is  too  small.  On 


0000 


(a) 


the  other  hand,  the  use  of  a  fuse  that  is  too  large  deprives 
the  car  of  the  very  protection  that  the  fuse  is  intended  to 
give.  With  a  fuse  of  the  right  size  a  car  would  never 
get  away  from  the  shop  with  the  fields  on  the  motor  con- 
nected wrong,  and  the  motorman  would  have  to  take  a 
loaded  car  up  a  grade  and  around  curves  on  the  series 
notch  of  the  controller  to  avoid  blowing  the  fuse.  With 
a  fuse  of  the  right  size  it  would  not  be  possible  for  a  car 
to  stay  in  service  for  weeks  at  a  time  with  its  fields  baked, 
calling  for  a  new  controller  or  brush  holder  or  armature 
once  a  week.  A  fuse  wire  may  be  said  to  be  of  the  right 
size  when,  under  normal  conditions  of  service,  it  will  blow 


INSTALLATION    CAR   EQUIPMENT   TESTS 


529 


in  about  two  weeks  after  being  put  in.  The  reason  it  is 
more  apt  to  melt  after  two  weeks  of  service  is  that  the 
heat  oxydizes  the  wire,  thereby  impairing  its  contacts. 

It  matters  not  what  kind  of  a  fuse-box  may  be  on  a  car, 
it  is  safest  to  throw  off  one  of  the  overhead  switches  be- 
fore renewing  a  fuse.  It  is  then  impossible  to  get  burned, 
shocked  or  have  the  car  start  unexpectedly.  To  get  a 
shock,  it  is  only  necessary  to  leave  the  pole  on  the  wire, 

N 


r—  —  1_£ 

j 

t 

r~ 

["LLSj" 

,  -, 

—  JLAJ 

1?9 

o 

j, 

j 

\ 

I 

I'       i 

-    ; 

/'             i«                • 

E 


FIG.  173. 

both  switches  in  and,  standing  on  the  ground  or  resting 
the  hand  on  any  part  of  the  truck,  dash  or  gate  irons,  to 
touch  the  positive  side  of  the  fuse-box.  To  get  burned 
under  like  conditions,  it  is  only  necessary  that  there  be  a 
ground  on  the  trunk  wire  between  the  fuse  and  the  con- 
troller trolley  post. 

Fig.  173  is  a  diagramatic  sketch  of  the  trunk  wire 
from  the  positive  side  of  the  fuse-box  to  the  negative  side 
of  the  blow  coil  on  one  controller,  and  to  the  trolley  post 
on  the  other.  Nc  is  the  wire  that  runs  from  the  No.  2 
overhead -switch  to  the  fuse-box,  FB;  LA  is  the  arrester; 


530  TESTING   OF   DYNAMOS   AND    MOTORS. 

//,  the  two  controller  trolley  posts,  and  tot,  the  car 
wire  that  connects  them ;  BC  is  the  blow  coil.  As  long  as 
the  condition  of  the  circuit  is  normal,  the  path  of  the  cur- 
rent is  Nc-FB-LA  to  the  tap,  x;  from  here  the  path  de- 
pends upon  which  controller  is  being  used.  If  A  is  used, 
the  path  from  x  is  x-o-t-BC-t';  on  through  the  controller 
and  motors  to  the  ground.  Suppose  that  a  car  under  head- 
way suddenly  develops  a  ground  on  the  trunk  wire  at 
any  of  the  points  indicated  by  the  dotted  lines, g^ ,  g2.  .  .  . 
£"5>  g*\  the  car  will  halt  in  its  speed;  if  on  a  grade,  it  will 
do  so  suddenly,  as  inertia  lends  less  influence  and  the 
blowing  of  the  fuse  cuts  off  the  ' '  power. ' '  The  motor- 
man  feels  the  check  in  the  speed,  and  throws  the  controller 
to  the  "off"  position,  which  act  can  make  no  difference, 
as  the  fault  is  ahead  of  the  moving  part  of  the  controller, 
and  therefore  cuts  it  out.  Such  a  fault  is  often  due  to  a 
grounded  blow  coil,  or  to  some  part  of  the  trolley  wire 
being  rubbed  by  a  brake-rod  or  car  wheel,  both  of  which 
are  dead  grounds.  An  old  motorman's  first  move  is  to 
try  his  car  lamps,  to  see  if  the  line  is  dead.  If  the  lamps 
light,  he  looks  at  the  fuse  to  see  if  it  is  gone,  and  in  this 
particular  case,  finding  that  it  is,  proceeds  to  renew  it. 
Once  in  a  while  the  fault  will  burn  itself  out  at  the  same 
time  that  the  fuse  does,  but  rarely.  The  fault  that  caused 
the  fuse  to  blow  in  the  first  place  is  ready  to  do  so  again. 
The  result,  then,  of  trying  to  replace  a  fuse,  with  the  pole 
on  and  switches  in,  under  any  of  the  above  conditions, 
would  be  to  have  the  fuse  blow  in  the  motorman's  hand  or 
face  and,  perhaps,  burn  him  badly. 

Now,  suppose  that  for  no  other  reason  than  that  the 
fuse  is  old,   it  gives  out  while  the  car  is  under  head- 


INSTALLATION   CAR    EQUIPMENT   TESTS.  531 

way,  and  that  the  motorman,  for  some  reason  or  other, 
fails  to  throw  his  controller  to  the  "off"  position  before 
getting  off  the  car.  As  soon  as  both  ends  of  the  new 
fuse  touch  the  blocks  the  car  starts.  In  nine  cases  out  of 
ten  the  motorman  will  get  a  shock  or  burn,  which  will 
place  him  in  very  poor  condition  to  catch  the  car  should 
the  fuse  catch  or  weld,  thereby  enabling  the  car  to  keep 
going. 

IV  THE  Lir.HTNiNr,  ARRESTERS. — The  fields  of  motors 
and  dynamos  are  powerful  electro-magnets  of  great  self- 
induction  and  on  some  of  them  it  takes  the  line  voltage  a 
quarter  of  a  minute  to  work  the  current  up  to  its  full 
value.  The  voltage  of  lightning  is  up  in  the  mil- 
lions and,  when  it  strikes  a  circuit,  something  must  break- 
down to  give  it  a  path  to  earth.  One  side  of  a  street  rail- 
way motor  is  always  grounded,  when  the  line  has  a 
ground  return.  The  path  from  the  trolley,  through  the 
motor,  to  the  rail,  tempts  the  lightning  and  it  moves  along 
this  path  smoothly,  until  it  gets  to  the  motor  itself,  where 
its  path  is  blocked  by  the  self-induction  of  the  motor  parts  ; 
but  the  discharge  with  such  an  enormous  pressure  behind 
it  can  not  be  withstood ;  it  must  get  to  earth  some  way, 
so  it  jumps  right  through  the  field  or  armature  insulation 
to  the  motor  frame.  The  path  through  the  motor  wind- 
ing is  an  inductive  path ;  the  path  through  the  insulation 
is  a  non-inductive  path.  Lightning  always  takes  a  non- 
inductive  path  where  it  has  a  choice,  but  where  there  is  no 
choice,  it  has  the  power  to  create  for  itself  a  non-inductive 
path  by  forcing  a  short  cut  through  the  inductive  path. 

In  Fig.  174,  T-a-G  is  a  piece  of  wire  bent  into  a  loop; 
one  end  sticks  up  in  the  air  at  T,  the  other  into  the  ground 


532 


TESTING  OF  DYNAMOS  AND  MOTORS. 


\ 


G 

FIG.  174. 


at  G;  o  is  an  air  gap;  T-o-G  is  a  non-inductive  path; 

T-a-G,  an  inductive  path.  If  by  means  of  an  influence 
machine  a  discharge  be  passed  into  the  wire 
at  T,  it  will  jump  the  gap  at  <?,  rather  than 
force  its  way  through  the  self-induction  of 
the  single  loop. 

The  function  of  a  lightning  arrester  is  to 
give  to  the  discharge  a  non-inductive,  low 
resistance  path  to  the  ground,  so  that  the 
motor  insulation  will  be  spared.  The 
arrester  humors  the  lightning  it  dares  not 
combat, 
In  Fig.  175,  T  is  connected  to  the  trolley  wire  and  G  to 

the  rail ;  A  and  F  are  the 

armature  and  field    of   the 

motor  to  be  protected  ;  L  is 

a  simple  form  of  aresrter, 

L    consists    of    two    brass 

plates  held  so  close  together 

that   the   air  gap   between 

them  is   thinner   than   the 

thinnest    insulation    to    be 

found  on  the   motor ;    one 

plate  connects  to  the  trunk 

wire  from  the  fuse-box,  the 

other  plate  to  the  ground. 

A  discharge  coming  in  at 

T,  jumps  the  gap  at  L  and 

goes  to  earth  at   G.     The 

arrester     outfit     shown    in 

Fig.    176  differs  from  that 


1       II 

v    * 

nr^ 

L 

A? 

AC 

2 

» 
i 

G                         |G 
FIG.  175.              FIG.  176, 

INSTALLATION   CAR    EQUIPMENT   TESTS. 


533 


in  Fig.  175  in  two  details: 
First,  the  two  plates  have 
comb-shaped  edges,  be- 
cause of  the  greater  dis- 
charging power  of  points; 
second,  it  has  coil  .9,  call- 
ed a  "kicking  coil,"  or 
"kicker."  The  kicker 
is  inserted  to  make  the 
motor  path  more  uninvit- 
ing to  lightning  on  the 
principal  that  lightning 
discharges  are  alternating 
in  character,  and  that  an 
inductive  resistance 
properly  placed  renders  it 


FIG.  178. 


FIG.  177. 

practically  im- 
possible for  the 
lightning  to  take 
that  path.  The 
kicker  is  noth- 
ing more  than 
about  a  dozen 
turns  of  the 
trunk  w  ire 
wound  on  a 
wooden  core.  All 
arrester  wires 
should  be 
straight  so  that 
their  self-indue- 


534 


TESTING  OP  DYNAMOS  AND  MOTORS. 


tion  will  be  a  minimum.  The  air  gap  in  an  arrester 
should  not  be  over  1-64  in.,  to  be  reliable  in  railway 
work.  The  500- volt  trolley  pressure  can  not  jump 
across  the  gap  in  an  arrester,  but  it  is  strong  enough 
to  follow  the  lightning  discharge  over.  Means  must  be 
taken  to  stop  the  arc  set  up  by  the  trolley  voltage,  or  the 
points  of  the  arrester  will  be  destroyed.  To  extinguish 


FIG.  179. 


FIG.  1 80. 


the  arc  that  follows  the  discharge  several  devices  have 
been  used.  The  one  that  has  perhaps  been  most  effective 
in  this  field  is  the  magnetic -blowout  incorporated  in  the 
arrester  of  the  Gen.  Klec.  Co. 

Figs.  177,  178,  179,  1 80  and  181  show  a  type  of  mag- 
netic blowout  arrester  that  has  taken  a  strong  hold  with 
the  railway  public.  Fig.  177  shows  the  general  appear- 
ance of  the  arrester,  and  Fig.  178  shows  it  in  a  box  ready 


INSTALLATION    CAR    EQUIPMENT   TESTS.  555 

t'o  be  put  on  a  car.  Fig.  179  shows  the  bare  arrester  with 
the  lid  off,  and  Fig.  180  shows  the  lid.  In  Fig.  179,  c  is 
ihc  blow  coil;  r,  a  non-inductive  resistance;  h,  h,  the 
magnet  poles,  and  k  /•,  the  knife  blades  that  enter  jaws 
magnet  poles,  and  A'A'.  the  knife  blades  that  enter  jaws 
K  K ^  Fig.  180,  of  the  lid  when  it  is  on.  The  air  gap.  a, 
Fig.  1 80,  is  only  .025  in.  wide.  The  air  gap  is  cut  into  cir- 
cuit when  the  lid  is  put  on,  and  it  falls  directly  between  the 
horns  of  the  blow  magnet,  so  that  any  arc  is  quickly  sup- 
pressed. The  clip,  /,  Fig.  180,  engages  knob,  /,  Fig.  179, 
and  holds  the  lid  in  place.  The  main  case  and  lid  are 
porcelain  ;  there  are  no  moving  parts.  The  gap  will  pass 
a  spark  to  earth  at  2000  volts.  The  spark  points  are  on 
the  lid  and  are  accessible  for  inspection,  renewal  or  ad- 
justment. In  series  with  the  spark  gap  is  a  non-inductive 
carbon  resistance  that  limits  the  value  of  the  line  current 
that  can  follow  the  spark  and  lessens  the  burning 
of  the  points.  In  order  that  the  blow  coil  may  offer  no 
inductive  resistance  to  the  discharge,  it  is  in  multiple  with 
a  part  of  r,  Fig.  1 8 1 ,  showing  the  connections,  z  I  is  a  car- 
bon resistance  divided  into  two  parts,  r'  and  r;  r'  is  in 
multiple  with  blow  coil,  d,  while  r  is  in  series  with  both ; 
one  end  of  the  blow  coil  connects  to  one  side  of  the  air 
gap,  and  to  one  end  of  the  carbon  resistance  at  z;  the 
other  end  of  the  coil  connects  to  the  resistance  at  point  p. 
the  junction  of  r  and  r1 .  The  trolley  comes  in  at  the  top 
and  connects  to  one  side  of  the  gap.  For  simplicity,  the 
air  gap  connections  are  shown  as  permanent,  but,  in  fact, 
the  air  gap  is  not  in  circuit  till  the  lid  is  on.  In  Fig.  181,  / 
is  the  wire  leading  to  the  fuse-box ;  t't  the  wire  from  the 
fuse-box ;  k  is  the  kicker,  and  /  and  a,  the  motor  field 


536 


TESTING   OF   DYNAMOS   AND    MOTORS. 


and  armature ;  g  is  the  motor  ground  wire,  and  g'  that  of 
the  arrester.  Confusion  of  the  trolley  and  ground  wires 
can  do  no  harm,  as  the  main  current  does  not  enter  the 
device  except  when  a  discharge  occurs.  Ordinarily,  the 
current  path  is  /,  t'-k-f-a-g ;  if  a  discharge  occurs  the 
path  is  t-t'-t"-a-n-z-p-r-l-g' .  The  trolley  current  that  fol- 
lows splits  at  n  ;  part  takes  the  path  n-z  r'-p,  and  the 
larger  part  takes  the  path  n-x-d-y  to  />,  through  the  blow 


coil.  The  lightning  discharge,  being  alternating  in  char- 
acter, avoids  the  blow  coil  on  account  of  its  self-induc- 
tion, and  takes  the  carbon  path ;  the  trolley  current 
being  direct,  avoids  the  carbon  on  account  of  its  high 
resistance,  and  takes  the  blow  coil  path.  Fig.  181 
shows  the  two  leads  to  come  out  at  the  bottom  and  top ; 
when  a  wooden  case  is  used  (Fig.  178)  both  leads  come 
out  at  the  bottom  of  the  case.  As  the  arrester  has  no 
moving  parts,  it  can  be  set  up  in  any  position.  The  de- 
vice is  giving  satisfaction,  notwithstanding  the  many  pre- 


INSTALLATION   CAR    KQUIPMENT   TESTS.  537 

dictions  that  the  porcelain  parts  would  never  stand  the 
heat  of  the  arc. 

POINTS  ON  ARRESTERS. — All  arresters,  whatever  their 
make,  should  be  inspected  after  each  storm;  even  if  the 
device  itself  is  in  good  order,  there  may  be  some  broken 
or  burnt-off  connection  leading  to  or  from  it.  An  open 
ground  or  trolley  wire  renders  the  device  inoperative. 
The  main  point  of  care  on  an  arrester  is  to  keep  the  air 
gap  thinner  than  any  of  the  insulation  the  device  is  to 
protect.  Only  constant  inspection  can  insure  this  adjust- 
ment, as  each  discharge  burns  the  gap  a  little  wider  and 
the  jolting  of  the  car  may  not  help  matters  any.  The 
space  between  the  points  must  be  kept  clean,  and  all  con- 
nections tight.  An  arrester  in  good  order  will  always 
protect  a  car  from  the  effects  of  mild  discharges  due  to 
the  static  induction  of  charged  clouds.  There  are  some 
bolts  of  lightning  against  which  there  can  be  no  pro- 
tection. If  from  static  causes  the  line  potential  rises  to 
several  thousand  volts,  an  arrester  in  good  order  will  pass 
a  spark  to  earth  and  relieve  the  tension ;  but  if  a  heavy 
bolt  of  lightning,  such  a  one  as  jumps  a  quarter  of  a  mile 
through  air,  splintering  a  tree  or  shattering  the  side  of  a 
house,  strikes  a  car,  the  arrester  is  by  no  means  a  cer- 
tain protection. 

The  insulation  on  street  car  motors  is  tested  to  only 
2000  volts  at  the  factory.  The  gap  on  an  arrester,  then, 
must  be  adjusted  to  pass  a  spark  before  the  line  reaches 
this  potential,  or  the  device  will  be  of  no  protection.  The 
insulation  of  motors  alternately  idle  and  active,  absorbs 
more  or  less  moisture  and  is,  therefore,  not  as  high  when 
old,  as  when  new. 


538  TESTING  OF  DYNAMOS  AND  MOTORS. 

It  would  be  a  simple  matter  to  make  an  arrester  ex- 
tremely sensitive,  if  the  lightning  were  the  only  factor 
to  contend  with ;  but  the  normal  current  that  follows  the 
discharge  on  a  high- voltage  circuit  must  be  cared  for 
or  it  will  burn  the  points  so  as  to  render  the  device  use- 
less for  the  next  discharge,  and  discharges  often  follow 
each  other  at  very  close  intervals.  Again,  the  two 
spark  points  may  weld,  so  that  the  arrester  ground  or 
trolley  wire  must  be  cut  before  the  car  can  move  itself  at 
all.  On  a  road  several  miles  long,  the  same  discharge 
has  been  known  to  injure  cars  at  opposite  ends  of  the 
line ;  this  accounts  for  the  fact  that  cars  are  sometimes 
struck,  when  no  thunder  or  lightning  are  in  evidence. 
Certain  conditions  raise  the  potential  of  the  whole  line, 
and  the  most  sensitive  arrester  or  the  weakest  insulation 
passes  the  first  discharge. 

It  has  been  found  impracticable  to  work  an  arrester  on 
an  air  gap  thinner  than  .025  in.,  which  is  thin  enough  for 
ample  protection,  as  none  of  the  insulation  of  the  motor 
or  controller  ever  approaches  this  value. 

V  THE  CONTROLLER,  (i)  RHEOSTAT,  RESISTANCE,  OR 
STARTING  COIL. — The  starting  coil  is  a  resistance  used 
to  limit  the  value  of  the  starting  current.  It  permits  the 
car  to  start  smoothly  and  saves  the  motors  undue  strain. 
In  connection  with  the  controller  it  regulates  the  speed 
after  the  car  is  started.  When  a  current  passes  through 
resistance,  heat  is  developed  and  energy  lost;  if  it  were  not 
for  these  facts,  the  motors  might  be  wound  to  give  resist- 
ance enough  to  limit  the  starting  current ;  but  this  resist- 
ance would  be  in  circuit  all  the  time  and  cause  a  constant 
loss  of  energy,  whereas  the  starting  coil  is  cut  out  on  cer- 


INSTALLATION   CAR    EQUIPMENT   TESTS  539 

tain  notches.  Motormen  have  had  occasion  to  observe  that 
some  cars  run  slower  after  having  made  several  trips.  The 
fact  is  most  noticeable  on  heavy  cars  equipped  with  old  style 
motors,  and  is  due  to  the  heating  which  raises  the  resist- 
ance of  the  motor  winding.  The  practice  of  to-day  is  to 
minimize  the  heating  and  loss  of  engery  in  the  motor  by 
making  the  resistance  of  the  winding  as  low  as  possible. 
In  order  to  keep  the  5OO-volt  line  pressure  from  sending 
an  abnormal  current  through  the  low  resistance  motors 
at  starting,  the  starting  coil  is  used.  It  is  true,  the  start- 
ing coil  gets  very  hot,  and  this  heat  represents  lost  en- 
ergy ;  but  the  coil  is  used  only  on  resistance  notches,  and 
does  not  affect  the  car's  maximum  speed. 

There  are  three  good  reasons  why  a  car  should  not  be 
run  for  any  length  of  time  "on  resistance."  First.  It  in- 
jures the  coil,  as  it  is  not  designed  for  continuous  run- 
ning. Second.  It  is  very  uneconomical,  the  heat  repre- 
senting so  much  lost  energy.  Third.  If  one  resistance 
notch  is  used  to  the  exclusion  of  others,  one  part  of  the 
coil  heats  excessively,  its  resistance  trebles  or  quadruples, 
and  causes  the  car  to  jump  badly  when  that  section  is  cut 
out  by  the  controller.  Ordinary  starting  coils,  when  used 
with  the  proper  sized  motors  and  cars,  will  about  double 
their  cold  resistance  and  hold  this  value  in  continuous 
normal  service.  But  when  the  coil  is  abused,  either  by 
the  motorman  or  the  management,  it  may  get  so  hot  as  to 
set  the  car  floor  on  fire.  A  very  hot  coil  is  certain  to 
make  the  car  jump  on  some  notches,  while  other  notches 
will  not  be  felt  at  .all. 

Resistance  coils  as  they  leave  the  factory  are  designed 
so  that  on  a  car  of  the  right  weight,  on  a  level,  equipped 


540          TESTING  OF  DYNAMOS  AND  MOTORS. 

with  the  motors  for  which  the  coil  is  intended,  the  car 
will  start  with  a  jerk  when  the  coil  is  cold,  but  will  set- 
tle down  to  smooth  starting  as  the  coil  warms  up. 

The  motors  and  controlling  devices  on  many  roads  are 
very  much  abused  in  the  following  manner :  A  lot  of 
motors  of  a  certain  size  are  bought,  for  example,  to  put 
under  i6-ft.  horse  cars  that  have  been  strengthened  for 
the  purpose.  The  cars  are  light,  the  road  level,  and  the 
runs  easy,  with,  perhaps,  a  lay  over  at  both  ends.  In 
course  of  time  the  traffic  grows,  the  road  is  extended,  the 
time-table  is  revised  and  two  small  cars  are  spliced  to- 
gether to  make  one  big  one.  Almost  before  it  is  realized, 
the  motors  that  handled  the  smaller  cars  so  well  under 
the  proper  conditions,  are  required  to  tug  an  "eight- 
wheeler"  over  5  to  10  per  cent  grades;  then  because 
they  revolt  at  such  abuse,  are  condemned,  and  new  ones 
bought  from  a  different  company. 

In  many  cases,  the  starting  coil  is  the  first  device  to 
show  the  effects  of  such  abuse.  As  soon  as  the  coils  be- 
gin to  open  circuit,  short  circuit,  roast  and  fall  to  pieces, 
they  are  replaced  by  coils  that  will  "stand  more  current" 
— coils  better  suited,  perhaps,  to  start  motors  of  twice  the 
size — with  the  result  that  the  car  starts  with  a  current 
that  ought  to  blow  the  fuse.  The  final  effect  of  such  a 
change  is  to  roast  the  motor  fields  and  blow  up  the  con- 
trollers. 

When  starting  coils  begin  to  give  general  trouble,  the 
idea  that  more  current  capacity  is  needed  is  correct,  but 
it  must  be  carried  out  with  the  condition  in  mind  that  the 
resistance  be  kept  the  same.  ,  To  fulfil  this  condition,  the 
coil  must  be  four  times  as  heavy,  when  the  current  ca- 


INSTALLATION   CAR    EQUIPMENT   TESTS.  541 

pacity  is  doubled,  because  not  only  is  the  cross-section 
of  the  resistance  wire  doubled,  but  the  length  is 
doubled  too.  Fig.  182  (a,  b,  c)  will  help  to  make 
this  idea  clear.  In  sketch  (a)  r  is  a  coil  of  given  resist- 
ance, say  two  ohms.  It  is  desired  to  combine  the  least 
number  of  these  coils  that  will  double  the  capacity  of  fa) 
without  changing  the  resistance.  The  resistance  from  .r 
to  y,  (a),  is  two  ohms:  that  from  .r  to  v  (b)  is  one  ohm, 
because  there  are  two  two-ohm  paths  in  multiple. 


•HWWWWWW 


Sketch  (b)  represents  the  condition  where  the  current 
capacity  has  been  doubled,  but  the  resistance  halved.  To 
restore  the  resistance  to  its  former  value,  two  ohms,  take 
two  units  like  that  in  sketch  (b)  and  connect  them  in 
series  as  shown  in  sketch  (c).  Combination  (c)  con- 
tains four  such  coils  as  r,  and  is,  therefore,  four  times  as 
heavy. 

Fig.  183  shows  a  substantial  form  of  coil  very  much  in 
use.  Fig.  1 84  shows  how  any  number  of  these  coils  can 
be  hung  under  a  car.  The  coil  is  built  up  of  band  iron 


542 


TESTING  OF  DYNAMOS  AND  MOTORS. 


and  mica,  and  up  to  certain  reasonable  limits  is  not  in- 
jured by  heat  or  water.  A  single  coil  is  called  a  barrel, 
and  the  proper  resistance  for  starting  any  railway  motor 


FIG.  183. 

can  be  made  up  by  combining  two  or  more  of  these  bar- 
rels in  series  or  multiple,  or  both. 

(2)  THE  SHUNT  AND  LOOP. — These  devices  are  used 
to  increase  the  speed  of  a  car.  They  have  some  virtues, 
but  their  failings  have  condemned  them  to  be  set  aside. 
As  has  been  shown  on  pages  51  and  52,  weakening  the 


FIG.  184. 

fiield  of  a  motor  increases  its  speed.  The  field  can  be  weak- 
ened either  by  shunting  some  of  the  field  current  or  by 
cutting  out  some  of  the  field  turns;  either  procedure  lessens 


INSTALLATION   CAR    EQUIPMENT   TESTS. 


thfc  ampere-turns.  The  shunt  does  the  former,  the  loop  the 
latter.  Fig.  185  is  the  wiring-  diagram  for  a  car  equipped 
with  one  motor,  one  old  style  rheostat  and  a  reverse 
switch,  A'.  T  is  the  trolley;  FB,  the  fuse-box;  LA, 
the  arrester;  R  the  resistance  which  is  cut  out  gradually 
as  the  shoe  B  moves  from  C  to  D.  Arm,  AB,  is  turned 
by  a  handle  in  the  driver's  control.  .-/-}-,  A —  is  the  arma- 
ture and  F-\-.F — ,  the  field.  £  is  the  shunt  of  low  resist- 


ance,  one  end  of  which  is  tapped  on  the  field  wire  at  O. 
The  other  end  goes  to  plate  E  on  the  rheostat.  To  start 
the  car  B  is  put  on  C  and  the  current  takes  path — T-FB- 
A-B-C-R-D-F+-F — O-A+-A—  to  the  ground  at  G.  As 
the  shoe  B  revolves  toward  plate  D,  R  is  gradually  cut 
out,  until,  when  B  touches  D,  all  of  A  is  cut  out  and  the 
current  path  is  T-FB-A-B-D-F+-F — O-A+-A — G. 
The  motor  is  directly  across  the  line  but  has  a  full  field, 
it  B  is  advanced  until  it  touches  both  D  and  £,  the  shunt 


544 


TESTING  OF  DYNAMOS  AND  MOTORS. 


becomes  connected  to  the  motor  field  at  both  ends  and  is  in 
multiple  with  it,  so  that  the  current  leaving  shoe  B, 

reaches  0  by  way  of  two  paths  :  B-D-F-\--F O  through 

the  field,  and  B-E-S-O  through  the  shunt.  Plates  D  and 
E  are  insulated  from  each  other  so  that  the  shunt  is  idle 
until  B  touches  both.  Were  D  and  E  connected  the  shunt 
would  be  active  throughout  from  C  to  £,  greatly  impair- 
ing the  starting  power  of  the  car.  An  open  circuit  in  the 


F- 


shunt  or  its  wire  simply  decreases  the  maximum  speed 
of  the  car. 

Fig.  1 86  is  the  same  as  185,  except  that  a  loop  replaces 
the  shunt.    When  B  touches  D,  the  current  path  is  T-FB- 

A-B-D-F+-F A+-A G.     When  B  touches  both  D 

and  E,  the  current  reaches  O  by  two  paths  :  B-D-N-F-\--O 
and  B-E-L-O ;  the  latter  is  a  short  circuit  so  that  the 
lower  part  of  the  field  is  cut  out.  In  shunt  control,  then, 
a  part  of  the  current  goes  through  all  of  the  field ;  in  loop 
control,  all  of  the  current  goes  through  a  part  of  the  field. 


INSTALLATION    CAR    EQUIPMENT   TESTS.  545 

An  open  circuit  in  cither  the  shunt  wire  or  L>op  wire 
does  not  affect  the  starting  of  the  car.  An  open  circuit 
in  the  D-F  -\-  field  wire,  Fig.  185,  will  make  it  im- 
possible to  start  the  car  at  all,  and  will  burn  the  shunt  out 
if  B  is  put  on  E  and  allowed  to  stay  there  a  few  seconds. 
In  Fig  1 86,  an  open  circuit  between  O  and  7; —  kills  the 
car ;  with  an  open  circuit  between  C  and  D,  the  car  is 
dead  until  B  touches  /:,  when  the  car  starts  with  a  jump. 
The  more  the  shunt  is  used,  the  cooler  the  fields  run,  for 
the  shunt  relieves  them  of  current.  When  the  loop  is  used 
continuously,  one  part  of  the  field  cools  off  and  the  other 
parts  gets  hotter,  because  one  part  is  cut  out  and  the  other 
part  gets  more  current  than  ever.  Shunts  and  loops  have 
been  set  aside,  because  from  a  practical  standpoint  their 
troubles  outweigh  their  advantages. 

(3)  CONTROLLER  PROPER. — Figs.  185  and  186  give  an 
idea  of  the  old  rheostat  formerly  in  general  use;  with  this 
device,  the  motors  were  permanently  connected,  either  in 
series  or  multiple.  Both  of  these  methods  have  given  way  to 
series-parallel  control.  In  series-parallel  control,  the 
motors  are  started  in  series  and  after  the  car  is  up  to  half 
speed  the  motors  are  thrown  into  multiple.  This  method 
requires  less  current  to  start  a  car,  because,  the  motors 
being  in  series,  each  gets  the  total  main  current.  When 
the  two  motors  are  started  in  multiple,  each  motor  gets 
but  half  the  total  current,  so  that  in  order  for  each  to  get 
a  certain  current,  the  total  current  must  be  twice  as  great 
as  when  the  motors  are  in  series.  This  means  that  the 
line  losses  are  four  times  as  great.  If  the  station  voltage 
is  500  and  the  line  drop  25  volts  when  starting  a  car  at  a  dis- 
tant point  with  the  motors  in  series ;  to  start  it  with  the  same 


546  TESTING  OF  DYNAMOS  AND  MOTORS. 

impulse,  but  with  the  motors  in  multiple,  calls  for  twice 
as  great  a  line  current,  which  causes  twice  as  great  a  drop, 
or  50  volts.  Since  the  current  and  line  drop  have  both 
been  doubled,  the  power  lost  in  the  line  has  been -quad- 
rupled ;  for,  call  E  the  volts  lost ;  C,  the  current ;  W,  the 
watts  lost.  In  the  first  case,  W^—E^C.  In  the  second 
case,  lVz~2Ey^2C=4EC—^W^  When  the  two  motors 
are  in  series,  their  C.E.M.F's  are  in  series,  and  hence  the 
spurious  or  non-heat  generating  resistance  is  much  great- 
er than  when  the  motors  are  in  multiple.  This  means, 
practically  speaking,  greater  economy  at  the  medium 
speeds,  because  the  C.  E.  M.  F's  take  the  place  of 
the  starting  coil  to  a  greater  extent,  and  diminish  the  heat 
losses  in  that  coil.  Of  course,  a  small  starting  coil  is  re- 
quired to  start,  because,  until  the  car  begins  to  move,  the 
C.E.M.F.  is  zero.  With  no  coil  at  all,  the  car  would 
start  with  a  jerk  on  good  rails  and  the  wheels  might  slip 
so  badly  as  to  prevent  its  starting  at  all  on  bad  rails.  A 
well  designed  and  well  handled  series-parallel  controller 
effects  an  economy  in  starting  and  on  the  low  and  medium 
speeds.  To  get  the  full  benefit  of  the  control,  though,  it 
is  necessary  to  let  each  controller  notch  have  its  full  effect 
before  moving  the  handle  to  the  next  one ;  also  to  allow 
the  motors  to  attain  their  full  series  speed  before  throw- 
ing them  into  multiple.  A  series-parallel  controller  can 
be  just  as  much  abused  as  any  other  kind  of  controller, 
and  when  it  is,  is  apt  to  give  less  economical  service  than 
some  of  the  older  types. 

Fig.  187  is  a  general  view  of  the  General  Electric  Com- 
pany's type  K,  magnetic  blowout,  series-parallel  con- 
troller. Fig.  1 88  gives  the  internal  controller  wiring; 


INSTALLATION    CAR    EQUIPMENT    TKSTS. 


547 


Fig.  '189  is  a  diagrammatic  sketch  of  the  connections  to 
the  motors.  J>  is  the  controller  drum,  made  up  of  five 
separate  castings  that  are  insulated  from  each  other.  The 


castings  have  tips  that  engage  the  fingers  successively  as 
the  drum  is  revolved.  Tips  alt  a*,  etc.,  belong  to  one 
casting,  and  tips  t^t  ^2,  etc.,  to  another  and  so  on.  The 
tips  are  so  marked  as  to  indicate  the  position  in  which 


548  TESTING   OF   DYNAMOS   AND    MOTORS. 


CONN-BOARD 


nan 


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i  i-1  1 

T-                                       (N 

Ul        O        _. 

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p 

F"0 

UJ 

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o> 

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UJ 

:nig_aj 

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LJ^nn 


a 


INSTALLATION   CAR    EQUIPMENT   TESTS. 


549 


they  come  into  action;  thus,  tips  </,  and  <-,    make  contact 
on  the  first  position;  tips  </,  0  on  the  tenth  or  last  position. 


MOTOR  NO.  1       MOTOR  NO.  2 

FIG.  189. 

B  is  the  reverse  drum  operated  by  handle  Z,  Fig.  187.  A 
car  must  be  so  connected  that  the  inclination  of  L  indi- 
cates the  direction  of  motion  of  the  car.  K  is  the  niDtor 


550  TESTING   OF   DYNAMOS   AND    MOTORS. 

cut-out  switch.  The  three  blocks  to  the  left  are  for 
motor  No.  i ;  the  two  to  the  right  for  motor  No.  2.  When 
both  switches  are  down  as  far  as  they  will  go,  as  in  Fig. 
187  both  motors  are  cut  in.  To  cut  a  motor  out,  its 
switch  is  thrown  up.  M  is  the  blow-out  coil ;  the  current 
from  the  fuse-box  enters  this  coil  first,  so  that  the  whole 
device  is  protected  from  arcs.  In  Fig.  189  FF'  and  A  A' , 
are  the  fields  and  armatures,  respectively,  of  the  Nos.  i 
and  2  motors ;  /  is  the  No.  i,  and  /'  the  No.  2,  loop  parts 
of  the  fields.  R  is  the  two-part  starting  coil.  The  top 
row  of  numbers  indicates  the  positions,  the  bottom  row, 
the  notches  of  the  drum.  The  blocks  to  the  left  of  the 
drums  represent  the  fingers. 

In  Fig.  189,  suppose  that  the  reverse  lever  points  ahead 
and  that  tips,  «/,  x,  y  and  2,  therefore,  engage  the  re- 
verse fingers.  When  the  power  drum,  D,  is  put  on  the 
first  notch,  the  current  path  is,  T-M-T-a^-a^-R^-r^-r^- 
7-3-19-19-19-2-^  -A-AA^  -y-F^-F-l-E^  -E^  -E^-E^-c^- 
£1-15-15- 15- i5-x-At-A'-AAirw-fls-irs-ftt-J7'-t'-G. 
The  blow  coil,  all  of  the  starting  coil  and  the  two  motors 
are  in  series.  On  the  second  notch  finger,  R%  touches  tip 
a%  and  short  circuits  the  part  of  the  starting  coil  that  lies 
between  r1  and  r2.  The  current  path  is  the  same  as  be- 
fore, excepting  that  R\,  r±  are  cut  out.  On  the  third 
notch  finger,  Jtz  touches  tip  a3,  and  cuts  out  the  rest  of 
the  resistance.  The  letters  ^3,  rz  can  now  be  left  out. 
The  blow  coil  and  two  motors  are  in  series  across  the  line. 
On  the  fourth  notch,  the  three  c±  and  e±  tips  fall  under 
fingers  L^ ,  Z2  and  G.  The  current  path  is  now :  T-M-  T- 


INSTALLATION    CAR    EQUIPMENT   TESTS.  551 

L^-e^-e^-G-G-G.  The  loop  wires  tap  on  to  the  field  coils 
of  the  two  motors  at  O  and  O',  when  the  current  gets  to 
these  points  it  takes  the  shorter  path  through  the  loop 
wires,  cutting  out  the  lower  part  of  both  fields.  The 
motors  are  in  series  with  the  blow  coil,  but  each  motor  has 
a  part  of  its  field  cut  out.  The  fifth  position  is  not  a 
notch,  and  the  current  path  is  the  same  as  on  the  second 
notch.  The  blow  coil,  half  the  starting  coil,  and  the  two 
motors  are  in  series.  Resistance  is  cut  in  again  to  les- 
sen the  current  before  passing  to  parallel.  On  the  sixth 
position,  the  two  </6  tips  touch  the  E^  and  G  fingers;  the 
current  path  is,  T-M-T-a^-a^-R^-r^-r^-i^-i^-i^-z-A ,- 
A- A  A  \-y-P \-F-l-E  ^-E \-E \-d \-J6-G.  In  passing  from 
the  fifth  to  the  sixth  position,  motor  Xo.  2  is  short  cir- 
cuited by  fingers  Ex  touching  tips  r,  and  </6  at  the  same 
time,  the  current  grounding  through  finger  G  and  lower 
tip  '/6 ,  so  that  on  the  sixth  position  motor  No.  2  is  dropped 
out  of  the  circuit  entirely.  The  blow  coil,  half  the  starting 
coil,  and  motor  No.  i  are  in  series.  The  seventh  position 
is  the  same  as  the  sixth.  On  the  fifth  notch  (eighth  po- 
sition) tips  £8  fall  under  fingers  19  and  15,  and  the  cur- 
rent path  becomes  :  T-M-T-a^a^R^-r^r^  :  ' ^"r9"T 9"g* 


,-,-,-t-t-. 

8  - 1 5- 1 5- 1 5- 1 5-*-^  2  -A'-AA  2  - w-F%  -Fz  -F*  -F'-l'-  G 
rs  the  current  splits;  part  takes  the  path  r3- 19- 19- 19-3 
through  motor  Xo.  i,  and  part,  path  *V-#3-i9-£8-£8,  etc., 
through  motor  No.  2.  The  blow  coil  and  half  the  resist- 
ance are  in  series  with  the  two  motors  which  are  in  mul- 
tiple with  ^ach  other.  On  notch  six  (position  nine),  -ffs 


552  TESTING  OF  DYNAMOS  AND  MOTORS. 

touches  tip  a9  and  cuts  out  the  starting  coil;  leaving  out 
It-g,  rz ,  the  path  of  the  current  is  the  same  as  above.  On 
the  seventh  and  last  notch  (position  ten),  tips  dl  0  touch 
fingers  Zt  and  Z2 ,  and  cut  the  loop  wires  into  action  again, 
cutting  out  the  lower  parts  of  both  motor  fields.  With  this 
exception,  the  current  path  is  the  same  as  on  the  sixth 
notch.  All  controllers  that  use  a  shunt  or  loop  have  two 
running  notches  in  series  and  two  in  multiple.  In  this  case, 
the  running  notches  are  three,  four,  six  and  seven.  Three 
is  used  for  climbing  grades;  four,  for  moderate  speed  on  a 
level;  six  and  seven,  for  high  speeds.  Four  and  seven 
should  never  be  used  for  climbing  grades.  When  one  motor 
is  cut  out  the  controller  is  operated  on  the  first  four  notches. 
The  throwing  of  either  cut-out  switch  so  disposes  an  in- 
terference that  the  controller  drum  can  not  be  moved  past 
the  fourth  notch,  the  controller  and  reverse  drums 
interlocking.  The  controller  drum  can  not  be  operated 
while  the  reverse  drum  is  at  the  "off"  position,  and  the 
reverse  drum  can  not  be  operated  while  the  controller 
drum  is  on.  This  feature  makes  it  impossible  for  the 
motorman  to  reverse  the  motors  without  first  throwing 
off  the  power,  and  it  also  gives  him  control  of  the  car 
when  he  may  not  be  on  it;  because  the  reverse  handle 
can  be  lifted  only  at  the  "off"  position,  so  that,  with  the 
reverse  handle  in  his  possession,  the  power  drum  can  not 
be  turned  by  any  one  who  does  not  understand  the  nature 
of  the  locking  device. 

(4)    THE  MOTORS. — Figs.  190  and  191  show  a  recent 
type  of  motor  made  by  the  Lorain  Steel  Company.     Fig. 

190  shows  it  complete,  ready  to  go  on  the  axle,  and  Fig. 

191  shows  the  case  open,  exposing  the  armature.     The 


INSTALLATION   CAR    EQUIPMENT   TESTS. 


553 


case  is  cast  in  two  halves  linked  together,  as  shown  at  ////, 
Fig.  191.  Of  low  carbon  steel,  it  is  stronger  than  cast 
iron  and  magnetically  nearly  as  good  as  wrought  iron. 
In  order  that  the  armature  or  a  field  coil  may  be  taken 
out  from  the  pit  without  disturbing  any  other  part,  all  up- 
to-date  motors  are  so  made  that  the  removal  of  a  couple 
of  bolts  on  each  end  will  permit  the  lower  casing  to  swing 


down,  either  with  or  without  the  armature.  Yokes,  YY, 
Fig.  191,  have  a  split  key  on  one  end  and  a  lock  nut  on 
the  other ;  one  shank  passes  through  a  lug  in  the  upper, 
and  the  other  shank  through  a  lug  on  the  lower  half  of  the 
case.  To  swing  the  armature  with  the  lower  case,  the 
split  keys  are  removed ;  to  retain  it  in  the  upper  case,  the 
keys  are  left  in  and  nuts  KK* ',  Fig.  190,  removed, DD, Fig. 
191,  are  oil  rings  that  turn  with  the  armature  and  throw 
all  grease  and  oil  down  on  the  track  through  the  aper- 
tures, one  side  of  one  of  which  is  seen  at  O.  Motors  on 


554 


TESTING  OF  DYNAMOS  AND  MOTORS. 


which  this  construction  is  used  are  said  to  have  "out- 
board" bearings.  The  grease  cups  are  over  the  bearings 
and,  are  cast  with  the  top  case.  The  bearings  are  malle- 
able iron  sleeves  lined  with  babbitt  and  provided  with 


FIG.   191. 

grooves  and  grease  holes;    they  are  centered  on  guide 
pins  that  keep  them  from  turning  with  the  axle. 

Fig.  192  shows  the  armature;  it  is  built  up  of  thin 
sheets  of  soft  iron,  pressed  together  and  held  by  steel  head- 
plates.  The  teeth  are  punched  in  the  plates  before  as- 
sembling, and  immersion  in  japan  insulates  the  plates 
from  each  other.  The  slots  are  big  enough  to  take  three 
coils,  so  that  there  are  but  one-third  as  many  slots  as  coils. 


INSTALLATION   CAR   EQUIPMENT   TESTS.  555 

This  lessens  the  cost  of  the  armature  and  strengthens  the 
teeth.  The  eoils  are  machine  wound ;  three  of  them  are 
bound  together  and  encased  in  an  insulating  armor,  which 
is  pressed  to  shape  in  a  press  through  which  steam  cir- 
culates ;  thirty-three  of  these  armored  coils  complete  an 
armature. 

In  the  connecting  of  the  armature  shown,  no  solder  is 
used ;  the  commutator  slots  are  milled  a  trifle  too  small 
for  the  leads;  the  slots  and  leads  are  tinned;  the  leads 


are  then  driven  into  the  slots  and  secured  by  a  strong 
band,  shown  at  5",  Fig.  192.  On  the  rear  of  the  coils,  is 
a  metal  shield,  F.  The  commutator  and  pinion  are  on  a 
tapered  seat.  The  commutator  is  of  unusually  liberal  di- 
mensions. 

Fig-  J93  shows  the  field  coil;  there  are  four  of  these 
to  slip  over  as  many  laminated  pole  pieces ;  the  pole  pieces 
are  detachable,  and,  when  drawn  home,  their  flanges  bear 
against  guide  ribs,  GG,  and  hold  the  coil  firmly  in  place. 
The  field  is  wound  on  a  double  flanged  shell,  which  ren- 
ders the  winding  immune  from  direct  pressure  of  any  kind, 
and  makes  it  impossible  for  the  pole  piece  to  cut  into  the 


556 


TESTING  OF  DYNAMOS  AND  MOTORS. 


corner  of  the  coil  or  for  the  shrinkage  of  the  insulation 
to  loosen  it.  This  manner  of  securing  is  positive.  O  and 
/  are  the  connecting  lugs ;  the  ends  of  the  coil  winding 
connect  to  these  lugs,  also  the  wires  used  to  connect  the 
coils  to  each  other  and  to  the  car  hose.  The  ridge  L  is 
a  pocket  cast  in  the  shell,  to  allow  the  inside  end  of  the 
winding  to  come  to  the  surface.  This  end  must  pass 

every  layer  of  wire  in 
the  coil,  and  is  extra 
well  insulated  to  insure 
safety.  The  end  is 
usually  brought  out  in 
the  form  of  a  terminal 
made  of  sheet  copper; 
this  flat  terminal  is 
soldered  to  the  end  of 
FIG.  193.  the  wire  and  insulated 

before  the  coil  is  begun. 

The  coil  shown,  as  well  as  all  recent  types  of  coil,  are 
rounded  off  on  one  side  to  fit  the  inside  lines  of  the  motor, 
so  that  there  is  little  chance  of  getting  the  coil  into  the 
motor  upside  down ;  but  on  most  all  coils  of  this  type,  it 
is  easy  enough  to  get  a  coil  in  end  for  end,  and  it  is  very 
often  done.  The  effect  of  such  a  mistake  is  to  bring  next 
to  each  other  in  the  motor  two  leads  that  should  not  con- 
nect together.  The  first  symptom  of  such  a  mistake  is 
that  the  brushes  spark  badly  on  the  series  notches,  and 
when  the  controller  is  thrown  over  to  multiple,  the  main 
motor  fuse  blows.  If  the  confusion  in  connecting  is  not 
bad  enough  to  continually  blow  the  fuse,  the  final  effect 
is  to  bake  the  insulation  on  the  field  wire.  It  is  possible 


INSTALLATION   CAR   EQUIPMENT   TESTS.  557 

for  the  field  coils  to  be  so  badly  confused  as  to  render  the 
motor  unable  to  start  at  all  alone,  but  if  there  are  two 
motors  on  the  car,  the  fault  may  not  be  suspected  until 
the  motors  are  thrown  into  multiple  and  the  fuse  blows. 
It  is  a  conservative  estimate  to  say  that  one-fifth  of  the 
baked  fields,  blown  fuses,  grounded  brush  holders  and 
knockcd-out  controllers  can  trace  their  failure  directly 
or  indirectly  to  wrongly  connected  field  coils.  Coils 
should  be  made  and  the  leads  brought  out  so  that  it  would 
be  impossible  to  get  one  in  upside  down  or  end  for  end. 
If  the  motor  makers  would  pay  half  the  attention  to  this 
detail  that  they  do  to  less  important  ones,  they  would  soon 
begin  to  reap  the  product  of  their  sowing. 


CHAPTER   XVI. 

CAR   EQUIPMENT   TESTS. 

Having  acquired  some  familiarity  with  the  devices 
which  go  on  a  car,  the  next  step  is  to  give  them  a  prelim- 
inary test  before  installing  them.  The  testing  of  the  trol- 
ley stand,  the  hood  switch  or  breaker,  the  fuse-box,  and 
the  lightning  arrester  have  been  considered,  so  that  it  is 
in  order  to  take  up  the  testing  of  the  resistance  coils,  the 
shunt,  the  controllers  and  the  motors. 

THE  TEST  CIRCUIT. — The  test  circuit  available  around 
a  car  house  usually  consists  of  a  magneto  bell  or  lamp 
circuit.  The  lamp  circuit  will  be  selected  in  this  case, 
because  its  indications  are  more  positive  and  it  can 
be  operated  by  one  man.  The  lamp  circuit,  as  a  rule, 
consists  of  a  single  row  of  five  lamps  in  series,  but  it  is 
much  more  satisfactory  to  use  two  five-lamp  rows  in  mul- 
tiple, as  shown  in  Fig.  194.  A  better  flash  is  gotten  on 
low  voltage,  and  if  one  row  gives  out  another  remains 
to  test  with.  This  lessens  the  liability  of  false  indications, 
due  to  the  tester  not  knowing  that  the  circuit  is  open. 
The  test  circuit  shown  on  Fig.  194  is  made  as  follows: 
take  two  boards  ^  in.  X  7  ins.  X  13  ins.  finished  off, 
groove  one,  as  shown  in  view  (c),  to  bury  the  connecting 
wires  shown  by  dotted  lines  in  view  (a)  ;  on  the  opposite 
side  from  the  grooves  mount  ten  keyless  lamp  sockets,  as 

558 


CAR   EQUIPMENT   TESTS. 


559 


indicated  by  the  dotted  circles  in  (c);  connect  these  all 
in  scries  and  bring  the  two  opposite  ends  to  posts  T  and 
T'  as  shown  in  the  figure.  The  second  piece  of  board  is 
now  screwed  to  the  back  of  the  grooved  one  to  protect  the 
the  connecting  wires.  The  whole  is  given  a  coat  of  thick 

-  T 


(a) 


?QL-CK>OL-Q 


(I) 


M£a 


FIG,  194. 

shellac,  such  as  comes  out  of  the  bottom  of  a  shellac  pot. 
After  this  is  dry,  the  device  is  ready  for  the  test  lines. 
The  test  lines  are  best  made  of  silk  covered,  rubber  cored, 
flexible  lamp  cord ;  each  line  is  a  complete  cord,  the  wires 
being  skinned  on  the  end,  twisted  together  and  dipped 
in  solder.  Both  ends  of  both  lines  have  a  terminal.  One 
terminal,  Fig.  194  ( d),  goes  to  the  post  T  or  7V.  It  is 
made  of  hard  spring  copper,  so  that  when  the  milled  head 


560  TESTING  OF  DYNAMOS  AND  MOTORS. 

is  screwed  down,  it  will  hold  the  terminal  securely.  The 
terminal  for  the  test  ends  of  the  lines  are  points  well  in- 
sulated, except  at  the  ends,  so  that  the  tester  may  not  be 
so  liable  to  a  shock.  The  point  on  one  line  is  straight 
and  on  the  other  is  both  straight  and  crooked,  to  hang 
over  the  trolley  wire,  or  to  stick  in  a  lamp  plug  switch. 
Fig.  195  shows  a  good  form  of  test  point.  T  is  a  brass 
rod,  pointed  at  one  and  drilled  at  the  other,  to  re- 
ceive the  test  line  which  is  soldered  in  place.  ^  is  a  hard 
rubber  or  red  fiber  tube  into  which  rod  T  is  forced  snug- 
ly. This  sleeve  extends  well  back  over  the  test  line  in- 


sulation, so  that  the  working  up  and  down  will  not  break 
the  wire  off. 

TESTING  THE  STARTING  COIL. — ^.  new  starting  coil 
must  be  tested  for  insulation  and  for  continuity.  In 
Fig.  183  the  coil  can  be  tested  for  insulation  as  follows: 
Lay  the  coil  down  so  that  one  of  the  end  nuts,  c,  touches 
the  rail ;  then,  with  the  test  lamps  connected,  as  in  Fig. 
196,  touch  any  of  the  lugs,  L,  L,  L,  with  the  test  point,  T; 
if  the  lamps  light,  it  shows  that  the  resistance  is  grounded 
to  the  rod  on  which  it  is  built  up.  If  the  lamps  do  not 
light,  touch  the  test  point  to  the  rail,  to  insure  that  the 
test  circuit  is  all  right.  If  the  circuit  is  all  right,  the  in- 
sulation is  O.  K.  To  test  for  continuity,  that  is,  to  see 
that  the  coil  has  no  open  circuit  in  it,  lay  the  coil  down  so 


CAR   EQUIPMENT   TESTS.  561 

that  one  of  the  end  plates  rests  on  the  rail,  and  touch  the 
other  end  plate  with  the  test  line.  If  the  lamps  light  the 
coil  is  O.  K.  If  they  da  not,  an  open  circuit  exists.  To 
find  out  in  which  end  of  the  coil  the  open  circuit  is,  lay 
the  coil  down  so  that  the  middle  lug  L  rests  on  the  rail, 
and  touch  both  end  plates  with  the  test  line.  The  open 
circuit  lies  nearest  the  end  plate  that  fails  to  complete  the 

TROLLEY  LINE 


TEST  LAMPS 


TC^O.,T| 
FIG.  196. 

circuit,  and  that,  therefore,  fails  to  light  the  lamps. 
Knowing  in  which  end  of  the  coil  the  break  lies,  a  great 
deal  of  labor  is  saved  in  unbuilding  the  coil  to  repair  it. 

TESTING  THE  CONTROLLER.  —  The  controller  must  be 
tested  for  grounds,  open  circuits,  and  short  circuits. 
Besides  this  electrical  test,  it  must  be  seen  to  that  the  in- 
terlocking device  and  motor  cut-out  switches  are  in 
working  order,  and  that  all  fingers  make  good  contact, 
and  are  in  alignment.  To  test  the  controller  for  grounds, 
lay  it  on  its  back,  so  that  the  iron  frame  rests  on  the  rail  ; 
see  that  both  drums  are  at  the  off  position,  that  both  cut- 
out switches  are  down  as  far  as  they  will  go,  and  that 


562  TESTING  OF  DYNAMOS  AND  MOTORS. 

the  ground  wire  that  runs  from  G  on  the  right-hand  cut- 
out, Fig.  1 89,  to  a  connecting  lug  on  the  bottom  bearing  of 
the  controller  drum,  is  disconnected.  (This  ground  wire 
is  used  to  prevent  the  controller  frame  from  ever  getting 
charged).  The  test  line  is  then  touched  to  all  parts  of 
the  controller,  one  at  a  time :  The  power  fingers,  reverse 
fingers,  cut-out  switches,  posts  on  the  connecting  board, 
all  the  castings  on  both  drums,  and  the  blow  coil.  If 
the  lamps  fail  to  light  under  all  these  tests,  the  insulation 
to  base  is  O.  K.  To  test  the  parts  for  cross  and  open 
circuits ;  the  test  lines  are  disposed,  as  shown  in  Fig.  197. 
Here  there  is  an  additional  test  line,  Tl  run  from  the 

TROLLEY 


RAIL 

TEST  LINE   J  *      T|  -TEST  LINE 
FIG.  197. 

rail.  If  these  two  test  lines  are  touched  together,  the 
lamps  light.  To  use  them  for  testing  the  controller,  one 
test  line  is  held  in  each  hand,  and  the  test  conducted  as 
follows:  (See  Fig.  188).  Holding  one  test  line  on  T 
finger  and  the  other  on  T  post  on  the  connecting  board, 
if  the  lamps  light,  it  shows  the  connections  from  T  to  T, 
through  the  blow  coil,  to  be  all  right.  Next  place  T  on 
finger  R^  and  7\  on  connecting  post  R^.  The  lamps 
should  light.  Fingers  R%  and  19  should  light  up  with 
cut-out  blocks  19,  reverse  finger  19,  and  connecting  post 


CAR    EQUIPMENT   TESTS.  563 

^?3.  Power  fingers,  -£", ,  should  light  up  with  cut-out  block, 
El.  The  four  armature  connecting  posts  light  up  with 
the  reverse  fingers  of  the  same  mark....  and  so  on. 
All  drum  tips  of  the  same  letter  (Fig.  189)  should  light 
the  lamps;  hut  where  the  test  lines  rest  on  tips  of  dif- 
ferent letters,  the  lamps  should  not  light. 

The  parts  of  the  controller  are  tested  for  short  circuit 
by  holding  a  test  line  on  one  part,  and  with  the  other 
test  line  touching  every  other  part  ;  the  lamps  should  in 
no  case  light  up  between  two  parts  that  the  drawing  does 
not  show  to  be  connected.  A  man  who  is  perfectly 
familiar  with  the  controller  he  is  testing,  can  test  it  in  a 
very  short  time. 

MOTOR  TESTS. — The  first  test  to  make  on  the  mo- 
tors is  the  insulation  test ;  this  is  done  by  setting  the 
motor  on  the  rail ;  this  grounds  the  frame.  L'sing  the 
connections  of  Fig.  196,  touch  the  test  line  to  all  of  the 
motor  leads,  one  after  the  other.  If  one  of  the  field 
leads,  say,  lights  the  test  lamps,  it  indicates  one  of  the  field 
coils  to  be  grounded ;  before  drawing  any  final  conclu- 
sion, see  that  the  opposite  end  of  the  field  whose  lead  is 
being  touched  is  not  resting  against  the  motor  frame ;  if 
the  field  circuit  proves  to  be  grounded,  disconnect  the 
back  field  connection,  thereby  separating  the  two  top 
fields  from  the  two  bottom  ones,  and  by  testing  them 
alone,  determine  in  which  half  of  the  motor  the  fault  lies. 
Having  determined  this,  open  the  motor  and  separate  the 
two  field  coils  in  the  faulty  half;  test  these  two  coils  one 
at  a  time,  and  take  out  the  grounded  one.  Very  often, 
upon  opening  the  motor,  the  ground  will  be  found  to  be 
due  to  a  lead  being  pinched  between  the  two  halves  of 


564  TESTING   OF   DYNAMOS   AND    MOTORS. 

the  case.  If  this  is  the  case,  it  will  not,  as  a  rule,  be 
necessary  to  remove  a  coil ;  this  pinched  lead  can  be  re- 
leased, spliced  if  neccessary,  taped  up  and  shellaced.  If 
one  of  the  armature  leads  shows  a  ground,  draw  the 
brushes  out  of  the  holders  and  test  both  leads  and  the 
armature.  If  the  drawing  of  the  brushes  removes  the 
ground,  the  armature  winding  itself  or  the  commutator 
must  be  the  faulty  member.  If  the  drawing  of  the 
brushes  does  not  remove  the  ground,  the  fault  must  be 
in  one  of  the  brush  holders.  This  is  not  certain,  though, 
for  the  brush  holders  may  have  dropped  down  far  enough 
to  touch  the  commutators  themselves,  in  which  case  the 
drawing  of  the  brushes  does  not  separate  them  from  the 
commutator.  If  the  fault  proves  to  be  in  the  armature 
itself,  it  must  be  taken  out,  the  leads  lifted,  and  each  coil 
tested  for  a  ground.  The  motors  are  always  tested  be- 
fore they  leave  the  factory,  but  in  spite  of  every  precau- 
tion, a  grounded  one  will  get  through  occasionally,  and  it 
is  very  unfortunate  if  it  finds  its  way  into  a  trial  equip- 
ment. In  the  test  and  on  a  car,  one  field  or  brush  holder 
on  one  motor  is  always  permanently  grounded;  if  the 
permanently  grounded  member  happens  to  be  defective, 
it  is  very  apt  to  get  out  on  the  road  in  that  condition. 

If  the  motor  fails  to  show  up  a  ground,  it  is  next  tested 
for  an  open  circuit,  as  follows:  First  take  a  field  lead, 
lay  it  on  the  rail  or  motor  frame,  and  lay  a  weight  on  top 
of  it  to  keep  it  grounded;  then  touch  the  other  end  of 
the  field  with  the  test  line.  If  the  lamps  light,  the  field 
circuit  is  O.  K.  Next  put  the  brushes  back  in  the  brush 
holders  and  test  the  armature  in  the  same  way,  seeing  to 
it  that  both  brushes  make  contact  with  the  commutator.. 


CAR    EQUIPMENT   TESTS. 


565 


Sometimes  shellac  on  the  commutator,  or  a  piece  of 
foreign  matter  under  the  brush,  will  keep  the  lamps  from 
lighting,  and  thereby  give  the  symptom  of  a  more  serious 
break. 

TESTING  THE  MOTOR  POLARITY. — Being  assured  that 
the  motors  are  neither  grounded  nor  open  circuited, 
the  next  step  is  to  get  the  polarity  tested  to  insure  that 


PINION  END 


PINION  END 


the  two  motors  shall  urge  the  car  in  the  same  direction 
after  they  are  mounted  on  the  truck.  The  motors  when 
swung  on  the  axle  are  back  to  back,  so  that  their  arma- 
tures must  turn  in  opposite  directions,  in  order  to  urge 
the  car  the  same  way.  To  get  the  polarity  right,  proceed 
as  follows  :  Set  the  two  motors  on  the  floor  back  to  back ; 


566  TESTING   OF   DYNAMOS   AND    MOTORS. 

their  commutators  will  then  point  in  opposite  directions, 
as  shown  in  Fig.  198.  Find  the  field  and  armature  leads 
on  the  two  motors.  To  be  certain  that  the  leads  have  not 
been  confused  in  bringing  them  out  of  the  motor  frame, 
run  the  hand  inside  the  motor  and  trace  them  from  their 
starting  point.  Mark  the  field  leads  with  a  piece  of  chalk 
and  leave  the  armature  leads  unmarked.  Assume  a  given 
direction  for  the  car  and,  therefore,  for  the  motors.  In 
Fig.  189,  it  will  be  seen  that  the  field  leads  on  the  No.  I 
motor  are  marked  JF^  and  E^ ,  and  on  No.  2  motor,  F% 
and  Ez  or  G.  The  No.  i  armature  leads  are  marked  A  ^ 
and  A  A 1,  and  the  No.  2  armature  leads,  A2  andAA2. 
The  Ez  lead  on  the  No.  2  motor  is  grounded,  so  that  on 
both  motors  the  fields  are  next  to  the  ground.  The  fields 
are  supposed  to  be  so  connected  that  the  current  enters  at 
their  F  ends  and  leaves  at  their  E  ends.  The  current  en- 
ters the  armatures  at  their  single  letter  ends  and  leave  at 
the  double  letter  ends.  Take  either  of  the  motors  as  it 
sits  on  the  ground  and  connect  one  field  and  armature  to- 
gether ;  ground  the  remaining  field  to  the  rail,  and  by 
means  of  a  wire  run  through  a  starting  coil  or  other  re- 
sistance, touch  the  remaining  armature  lead  and  give  the 
armature  a  spin.  If  it  turns  in  the  right  direction,  mark 
the  armature  lead  where  the  current  enters,  Ai,  the  other 
armature  lead,  A  A  i;  the  grounded  field  lead,  El ;  the 
other  field  lead,  Fi.  Should  the  armature  turn  in  the 
wrong  direction,  let  the  grounded  field  and  its  mate  ex- 
change places,  and  mark  them  accordingly.  The  No.  2 
motor  is  tested  and  marked  in  the  same  way.  Now  the 
taps  out  of  the  car  wiring  hose  have  the  same  marks  on 
them,  and  like  marks  connect  together.  The  final  result 


CAR    EQUIPMENT   TESTS.  567 

of  the  test  is  that  the  current  enters  the  two  motor  ar- 
mature leads  at  like  ends  of  the  motor ;  but  it  enters  the 
front  field  lead  on  the  No.  I  motor,  and  the  rear  field 
lead  on  the  No.  2  motor.  This  polarity  test  is  often  de- 
ferred until  the  two  motors  have  been  hung  on  the  car 
axles.  It  is  just  as  well  one  time  as  another.  It  is  nec- 
essary to  mark  the  leads  for  only  one  equipment,  by  test, 
when  the  leads  on  all  the  others  can  be  marked  similarly. 
A  man  familiar  with  the  motors,  controllers  and  car  wir- 
ing of  a  given  equipment,  does  not  need  any  marks  at  all. 

The  motors  being  electrically  all  right,  are  ready  to  hang 
on  the  axle.  At  the  same  time  that  the  motors  are  being 
mounted,  the  carpenter  is  setting  up  the  controllers,  in- 
stalling the  trolley  stand,  overhead  switches,  etc.,  so 
.that  all  branches  of  the  work  are  ready  at  about  the  same 
time,  and  the  car  can  be  set  down  on  the  truck.  The  car 
wiring  hose  is,  of  course,  made  up  and  installed  before 
the  car  body  is  let  down.  The  truck  boxes,  and  motor 
axle,  and  armature  boxes  are  oiled  or  packed  before  the 
truck  is  moved.  The  most  particular  part  of  the  truck 
work  is  the  setting  of  the  gear ;  the  gear  is  to  the  truck 
what  the  commutator  is  to  the  motor. 

SETTING  THE  GEAR. — To  put  on  a  gear  properly 
is  more  of  a  job  than  some  people  trunk  it  is.  It  will  be 
assumed  that  all  the  gear  teeth  are  the  same  size,  and  that 
the  two  halves  of  the  gear  are  mates ;  that  is,  that  they 
have  been  bored  together,  and  that  the  hole  is  in  the  center. 
The  first  thing  to  do  is  to  inspect  the  gear,  to  see  that  it 
is  of  the  size  and  bore  wanted ;  that  it  has  no  irregular 
teeth ;  that  the  teeth  have  not  been  hammered  on  the  end, 
making  a  ridge  to  bend  the  armature  shaft ;  that  it  has  no 


568  TESTING   OF  DYNAMOS  AND   MOTORS. 

cracks  visible  to  the  eye,  or  audible  when  the  gear  is  struck 
with  a  hammer.  If  there  are  any  ridges,  they  must  be 
chipped  and  dressed  with  a  file.  The  next  step  is  to  lay 
the  gear  down  and  take  the  bolts  out ;  never  use  a  monkey 
wrench  on  gear  nuts  ;  use  a  socket  wrench  or  an  S  wrench. 
When  the  gear  comes  apart,  sometimes,  though  not  al- 
ways, four  little  strips  of  sheet  iron,  called  shims  or  liners, 
fall  out.  The  gears  are  cut  with  these  shims  in  between 
the  two  halves,  with  the  result  that  both  halves  are  a  lit- 
tle less  then  half  a  circle,  so  that  when  the  gear  is  put  on 
an  axle  that  is  slightly  too  small,  it  will  pull  up  tight. 
The  position  of  the  keyway  for  the  key  that  secures  the 
gear  must  be  selected  with  due  regard  for  the  dimensions 
of  the  motor,  or  the  result  may  be  to  have  one  end  of  the 
motor  interfere  with  one  of  the  car  wheels.  The  key. 
should  be  fit  so  snugly  in  the  keyway  that,  when  struck 
with  the  hammer,  the  sound  and  feel  should  be  the  same  as 
if  the  axle  were  struck.  One  may  then  be  certain  that  the 
key  is  in  solid  and  not  apt  to  get  loose.  Before  inserting 
the  key,  all  burrs  should  be  smoothed  off  with  a  file,  and 
the  key  tried  in  both  the  axle  and  gear  keyways.  The  key 
snugly  in  place,  the  keyway  half  of  the  gear  is  put  on. 
If  the  gear  fits  the  axle  as  it  should,  it  should  require 
some  pounding  to  force  it  into  place.  Never  pound  a 
gear  with  a  hammer  without  laying  a  piece  of  copper  or 
wood  on  top  of  it.  If  pounding  fails  to  force  the  gear  on 
it  may  be  bored  too  small,  or  the  axle  may  be  a  trifle 
large.  In  such  a  case,  ascertain  which  is  at  fault  and 
remedy  it,  with  the  end  in  view  of  keeping  everything  to 
standard  size.  If  the  gear  fits  the  axle  exactly,  it  is  best 
to  put  a  piece  of  thin  paper  in  between  the  two  and  force 


CAR   EQUIPMENT  TESTS.  569 

the  gear  on  over  it ;  the  gear  is  then  much  less  apt  to  get 
loose.  In  some  cases,  where  a  new  gear  is  put  on  an 
old  axle,  it  sometimes  goes  on  so  loosely  that  it  is  ne- 
cessary to  put  in  a  liner  of  iron  or  emery  cloth.  Where 
a  gear  is  bored  too  small,  it  can  be  rebored,  but  where  it 
is  too  large,  if  it  must  be  used,  it  must  be  shimmed.  It 
is  even  the  practice  to  put  4  in.  gears  on  3f  in.  axles ;  but 
in  such  cases,  the  lining  must  be  done  with  great  care,  and 
the  key  must  be  made  deeper  to  allow  for  the  thickness  of 
the  liner.  When  a  liner  or  shim  gets  to  be  3-16  in.  thick, 
it  is  called  a  bushing,  and  the  best  way  to  make  such  a 
bushing  is  as  follows :  Take  a  wrought-iron  ring  of  the 
width  of  the  gear,  and  of  such  thickness  that  the  outside 
can  be  turned  to  fit  the  bore  of  the  gear,  and  the  inside  to 
fit  the  axle.  The  ring  is  put  in  a  lathe  and  finished  inside, 
so  it  can  be  driven  on  a  mandrel  and  finished  outside.  A 
piece  is  then  sawed  out  the  width  of  the  key,  and  the  ring 
split  to  go  on  the  axle.  A  gear  bushed  in  this  way  will 
give  no  trouble. 

If  the  upper  half  is  a  good  fit,  the  lower  half  is  very 
apt  to  be.  The  upper  half  is  the  more  particular  of  the 
two,  because  it  has  the  key  seat  in  it.  As  soon  as  the 
upper  half  is  on,  the  axle  is  turned,  the  lower  half  put  on 
and  the  bolts  put  in.  On  all  modern  gears  there  are  lugs 
cast  to  keep  the  bolt  heads  from  turning  when  tightening 
up  the  nuts.  Lock  washers  are  put  under  the  nuts,  so  that 
they  can  not  work  off  and  get  between  the  gears  and  bend 
the  armature  shaft  or  break  the  motor  frame.  In  tighten- 
ing up  the  bolts  in  a  gear  the  four  nearest  the  axle  should 
be  tightened  up  first.  When  a  gear  is  on  right,  the  face 
of  the  hub  should  square  up  with  the  axle.  The  better 
the  job  done  on  a  gear,  the  longer  it  will  last. 


570  TESTING   OF   DYNAMOS   AND    MOTORS. 

MOUNTING  THE  MOTORS. — The  gears  on,  it  is  in 
order  to  hang  the  motors.  Before  doing  so,  pour  a  little 
oil  in  the  armature  boxes  and  give  the  armature  a  spin, 
to  -see  that  the  armature  has  end  play,  that  none  of  the 
bearings  bind,  and  that  the  core  does  not  rub  the  pole 
pieces.  The  motor  must  be  so  placed  that  when  one  axle 
bearing  is  snug  against  the  gear  and  the  other  against 
the  axle  collar,  the  suspension  lug  will  fall  in  line  with 
its  support,  so  that  it  will  not  be  necessary  to  bar  the 
motor  over  and  put  a  wearing  strain  on  any  part  of  it. 

TESTING  THE  GEARS. — To  test  the  gears,  jack  the 
truck  up  on  one  end  until  the  wheels  on  that  end  are  off 
the  rail ;  also  block  up  the  motor  on  that  end,  to  take  its 
weight  off  the  axle,  because  the  car  journal  brass  is  on  top 
of  the  axle,  and  as  soon  as  the  truck  frame  is  raised,  the 
weight  of  the  motor  and  wheels  draws  the  axle  down  on 
the  bottom  of  the  journal  box,  where  there  is  no  bearing, 
and  where  the  axle  is  apt  to  be  badly  cut.  Next  connect 
up  the  motors  as  in  the  test  for  polarity,  and,  if  possible, 
insert  a  water  rheostat,  so  that  the  speed  can  be  regulated. 
The  water  resistance  is  then  adjusted  so  that  the  car 
wheels  make  about  150  r.  p.  m.  Any  fault  in  the  gear- 
ing will  manifest  itself  in  any  of  several  ways.-  The  gears 
are  first  given  a  spin  without  the  gear  case  on.  If  the 
gear  emits  a  grinding  noise  once  per  revolution  of  the 
car  wheel  it  indicates  that  the  car  axle  is  sprung  ;  or 
that  the  gear  is  bored  out  of  center;  or  that  the  gear  is 
put  on  lop-sided.  If  the  axle  is  sprung,  the  whole  motor 
will  be  seen  to  give  back  and  forth,  and  up  and  down, 
every  time  the  axle  revolves,  and  the  side  frames  of  the 
truck  will  also  be  seen  to  work  up  and  down.  If  the  gear 


CAR   EQUIPMENT   TESTS.  571 

is  bored  out  of  center,  the  fact  is  most  readily  detected  In- 
putting a  little  stiff  grease  on  the  gears  and  noting  how  it 
wears ;  on  the  high  side  of  the  gear  the  grease  will  work 
further  down  into  the  trough  of  the  teeth  than  on  the  low 
side ;  also,  one  familiar  with  the  work  is  able  to  recognize 
a  difference  in  the  sound  when  the  high  side  passes  under 
the  pinion.  If  the  gear  is  on  lop-sided,  it  will  be  seen  to 
wobble  when  looked  at  from  the  rear.  If  the  grinding 
noise  occurs  several  times  per  revolution  of  the  car  wheel, 
it  indicates  an  imperfection  in  the  pinion,  or  a  bent  ar- 
mature shaft.  If  the  shaft  is  bent,  the  motor  brushes 
will  chatter. 

A  clicking  or  knocking  noise  once  per  revolution  of  the 
gear,  indicates  that  the  gear  has  an  odd  sized  tooth  in 
it  (due  to  the  machine  or  to  the  man  that  cut  it)  ;  or  that 
the  two  halves  of  the  gear  are  open  on  one  side ;  or  the 
gear  may  be  loose  on  the  axle;  or  the  key  may  be  loose 
or  worn.  If  the  gear  has  one  or  more  big  teeth  in  it,  their 
wear  will  show  it,  and  the  trouble  can  be  relieved  by 
dressing  the  faulty  teeth  with  a  chisel  and  file.  If  the 
gear  has  several  small  teeth,  the  best  thing  to  do  is  to 
change  the  gear.  If  one  side  of  the  gear  is  open  on  one 
side,  close  it,  if  possible,  by  drawing  up  on  the  bolts ;  if 
not,  take  the  gear  off  and  set  it  again.  If  the  gear  or  key 
is  loose,  the  chances  are  that  both  key  and  keyway  are 
badly  worn,  and  the  best  thing  to  do  is  cut  a  keyway  on 
the  other  end  of  the  axle,  in  which  case  the  axle  must  be 
turned  end  for  end,  to  bring  the  gear  on  the  right  side. 
A  loose  gear  can  be  detected  by  pinching  the  gear  back 
and  forth  with  a  bar. 

If  the  gears  run  smoothly  with  the  cases  off,  put  the 


572  TESTING  OF  DYNAMOS  AND  MOTORS. 

cases  on  and  try  them  again.  Any  grinding  noise  will  be 
due  to  the  gear  or  pinion  rubbing  the  gear  case.  Anyone 
familiar  with  the  noises  can  tell  whether  it  is  the  gear  or 
pinion,  by  the  pitch  of  the  noise.  To  minimize  the 
chances  of  the  gear  case  rubbing,  all  burrs  should  be 
cleared  off  inside  of  both  ends  before  the  gear  case  is  put 
on.  The  gear  case  should  be  put  on  centrally,  or  the  side 
play  in  the  armature  will  let  the  pinion  over  against  it, 
and,  in  course  of  a  short  while,  the  pinion  will  mill  a  hole 
in  it. 

STARTING  THK  CAR. — The  controlling  devices  in- 
stalled, the  wiring  hose  in  place,  and  the  motors  mounted, 
the  truck  is  run  under  the  car  and  all  connections  made. 
If  everything  is  all  right,  the  car  is  ready  to  start  as  soon 
as  the  pole  is  on,  the  overhead  switches  closed,  and  the 
controller  put  on  the  first  notch.  It  is  a  wise  precaution 
when  installing  a  car  for  the  first  time  to  insert  a 
resistance  in  the  circuit  somewhere,  so  that  in  case  a 
ground  or  other  wrong  connection  exists  at  the  time  of 
trying  to  start,  there  will  be  no  violent  demonstration. 
As  good  a  place  as  any  to  insert  this  resistance  is  on  top 
of  the  car,  between  the  roof  wire  and  the  trolley  stand. 
Take  a  starting  coil  and  lay  it  on  a  sheet  of  ^-in.  asbestos, 
so  that  in  case  it  gets  hot,  the  car  roof  will  not  be  dam- 
aged. Everything  ready,  one  controller  is  put  on  the  first 
notch  (the  pole  being  on  and  the  hood  switches  in).  If 
the  car  fails  to  start,  it  may  be  due  to  absence  of  power  on 
the  line,  to  an  open  circuit,  or  to  a  ground  or  wrong  con- 
nection. If  the  controller  shows  no  flash  when  thrown 
off,  the  trouble  is  due  to  lack  of  power  or  to  an  open  cir- 
cuit. If  the  power  is  off  the  line,  the  car  lamps  will  fail 


CAR   EQUIPMENT  TESTS. 


573 


574  TESTING  OF  DYNAMOS  AND  MOTORS. 

to  burn,  and  so  will  the  test  lamps.  The  car  lamps  can 
not  be  relied  on,  alone,  for  the  lamp  circuit  may  be  de- 
fective. If  the  lamps  burn,  the  trouble  is  due  to  an  open 
circuit.  Try  the  controller  on  the  other  end  of  the  car; 
if  the  car  still  fails  to  start  there  also,  cut  in  one  motor  at  a 
time  and  try  both  motors;  if  the  car  still  fails  to  start,  the 
open  circuit  must  be  in  some  part  of  the  circuit  common 
to  both  motors,  anywhere  from  L  to  S,  Fig.  199,  or  in  the 
ground  wire,  and  may  be  due  to  any  of  the  following 
causes :  The  trolley  wheel  may  be  resting  on  a  line 
breaker;  an  overhead  switch  may  be  open  or  defective; 
there  may  be  too  thick  a  coat  of  paint  on  the  base  of  the 
pole,  insulating  it  from  the  socket ;  the  roof  trolley  wire 
may  be  disconnected  from  the  stand ;  the  lightning  ar- 
rester may  be  connected  in  wrong;  the  ground  connec- 
tion may  be  bad ;  the  car  may  be  standing  on  a  dead  rail. 
The  first  step  to  take  is  to  see  that  the  pole  is  on  the  wire, 
that  both  hood  switches  or  circuit  breakers  are  closed — 
they  are  supposed  to  have  been  tested  before  being  in- 
stalled— and  that  the  fuse-box  has  a  fuse  in  it.  If  these 
are  all  right,  inspect  the  trolley  stand  wire  and  ground 
wire.  Other  causes  are  harder  to  locate,  and  will  be 
deferred  until  the  lamp  test  is  taken  up. 

If  the  car  will  start  on  both  motors,  on  one  controller, 
but  will  not  start  at  all  on  the  other,  it  indicates  the 'open 
circuit  to  be  local  to  one  controller  circuit.  In  Fig.  199, 
an  open  circuit  from  tap  5  to  finger  ^  on  either  end, 
will  disable  the  car  on  that  end.  For  example,  if  one  of 
the  blow  coil  connections  gets  loose,  or  if  the  trolley 
finger  fails  to  make  contact  on  either  controller,  no  cur- 
rent can  get  to  either  motor,  so  the  car  can  not  be  operated 
from  that  end. 


CAR    EQUIPMENT  TESTS.  575 

If  the  car  fails  to  start  on  both  ends  with  both  motors 
cut  in,  but  will  start  on  both  ends  with  one  motor  cut  out, 
it  goes  to  show  that  the  break  is  local  to  the  motor  that  is 
cut  out.  One  of  its  leads  may  be  pulled  out  of  its  connect- 
or, or  a  brush  may  be  missing,  or  a  brush-holder  spring 
may  not  be  resting  on  the  brush.  In  any  case,  the  car  will 
not  start  with  the  motors  in  series,  but  will  start  on  the 
good  motor,  as  soon  as  the  controller  is  thrown  to  the 
multiple  position. 

With  an  open  circuit,  anywhere  from  finger  A',  to  the 
A*,  splice  of  the  starting  coil,  or  if  the  A',  finger  fails  to 
make  contact  with  the  drum,  the  controller  affected  will 
not  start  on  the  first  notch,  but  will  start  on  the  second. 
If  the  break  is  in  the  A*,  tap  wire,  or  in  the  starting  coil 
itself,  between  A*,  and  Rz  the  car  will  not  start  on  the 
first  notch  at  either  end.  An  open  circuit  on  the  A*3  tap, 
or  in  the  coil  itself,  between  A*2  and  A'.,  will  prevent  the 
car  from  starting  on  either  end  until  the  controller  is  put 
on  the  third  notch.  The  most  common  cause  of  failure 
to  start  is  want  of  contact  between  a  finger  and  the  drum. 
If  the  R»  finger  fails  to  make  contact  on  one  controller  the 
car  will  start  on  the  first  notch  of  that  controller,  but  will 
loose  the  power  on  the  second  notch,  and  pick  it  up  again 
on  the  third  notch.  If  the  break  is  in  the  R»  tap,  both 
ends  of  the  car  will  behave  as  in  the  last  case.  Any  trouble 
due  to  bad  finger  contacts  is  always  accompanied  by  more 
or  less  sizzling  that  can  be  heard  and  felt  in  the  handle. 

Some  open  circuits  can  be  readily  located  by  inspec- 
tion or  symptom,  and  others  can  not.  Any  kind  of  an 
open  circuit  can  be  quickly  located  with  a  test  lamp  cir- 
cuit, as  follows :  Hang  one  of  the  lamp  test  lines  over 


576 


TESTING   OF   DYNAMOS   AND   MOTORS. 


the  trolley,  or  stick  it  into  the  trolley  side  of  the  car  lamp 
circuit  leaving  the  other  test  line  free  to  test  with.  L,et  us 
assume  that  the  positive  test  line  is  hung  over  the  trolley 
wire.  In  this  case,  the  car  pole  is  tied  down,  the  over- 
head switches  put  in,  and  one  of  the  controllers  put  on  the 


FIG.  200. 

first  notch.  Suppose  that  the  open  circuit  is  due  to  paint 
on  the  bottom  of  the  pole.  The  only  way  the  lamp  circuit 
can  get  a  ground,  and  light  its  lamps,  is  by  way  of  the 
main  motor  circuit,  and  if  there  is  a  break  in  this  circuit 
the  lamps  can  not,  of  course,  light,  until  the  test  line  gets 
on  the  ground  side  of  the  break.  Accordingly,  the  lamps 


CAR   EQUIPMENT   TESTS.  577 

will  not  light  when  the  test  line  is  touched  to  the  trolley 
harp  or  wheel,  because  they  are  on  the  positive  side  of  the 
break ;  but  as  soon  as  the  test  line  touches  the  trolley 
stand,  or  the  roof  wire  running  to  it,  the  lamps  light. 

The  idea  is  more  clearly  seen  by  means  of  Fig.  200.  L 
is  the  test  bank  ;  P  is  the  pole ;  A',  the  socket ;  S,  the 
stand;  K,  K,  the  two  hood  switches;  AA',i\\c  two  mo- 
tors. The  break,  caused  by  the  paint,  is  between  P  and 
N.  When  the  test  line  is  touched  to  P,  the  lamps  do  not 
light,  because  no  current  can  get  through  the  paint ;  but 
when  the  test  line  is  touched  to  5",  the  current  can  pass 
through  the  path,  T-L-S-K-K-A-A' ,  to  the  ground  at  G. 

Suppose  the  break  to  be  due  to  a  wrong  connection  of 
the  lightning  arrester;  on  that  type  of  arrester  through 
whicn  the  motor  current  does  not  flow  (the  type  indi- 
cated in  Fig.  199),  there  is  little  chance  of  a  wrong  con- 
nection being  made;  such  arresters  have  but  two  leads, 
and  these  are  of  such  light  wire  that  no  one  with  common 
sense  would  connect  the  arrester  in  series — the  only  con- 
nection that  can  make  an  open  circuit ;  but  on  arresters 
that  have  three  connecting  wires  or  posts,  it  is  an  easy 
matter  to  create  an  open  circuit.  In  Fig.  201,  views  A 
and  B  show  two  three-post  arresters  properly  connected; 
a  is  the  air  gap  in  all  the  views.  C  and  D  show  the  same 
arresters  so  connected  that  the  trolley  wire  is  on  the 
wrong  side  of  the  air  gap.  In  this  case,  the  current  can 
not  get  to  the  motor  circuit,  as  there  is  a  break  at  a,  so 
the  test  lamps  can  not  light  until  the  test  line  crosses  the 

gap- 

In  Fig.  199  suppose  that  one  of  the  No.  i  motor 
brushes  is  left  out — the  A  l  brush,  for  example — touch  the 


578 


TESTING   OF   DYNAMOS   AND   MOTORS. 


test  line  successively  to  the  Tf  Jfl,  ^?2>  -^s>  ^i  and 
connecting1  posts ;  the  lamps  will  not  light  until  the  test 
line  touches  the  AAi  post,  showing  that  the  fault  lies 
somewhere  between  A^  and  A  A  j  Its  location  can  be 
still  further  determined  by  touching,  successively,  the  A^ 
reverse  finger  and  brush  holder,  and  the  AA  t  brush 
holder.  The  lamps  will  light  on  AAlt  but  not  on  Aly 

©T 


r 

f 


showing  the  fault  to  be  between  the  two.  In  all  cases, 
when  testing  for  an  open  circuit  with  the  lamp  circuit, 
the  fault  always  lies  between  the  two  points  between 
which  the  lamps  light  and  fail  to  light.. 

There  is  no  condition  that  will  give  a  novice  more 
trouble  than  a  "dead  rail,"  which  gives  the  same  symp- 
tom as  an  open  circuit.  A  dead  rail  is  due  to  the  break- 
ing of  the  bond  wires  that  connect  the  rails  together.  In 


CAR   EQUIPMENT  TESTS.  579 

such  a  case  the  open  circuit  exists  between  the  dead  rail 
and  the  rails  adjacent  to  both  ends  of  it.  A  dead  rail  is 
called  dead  because  no  current  can  get  from  it  to  adjoin- 
ing rails ;  but  it  gets  very  much  alive  when  a  car  runs 
on  to  it,  and  a  person  standing  on  an  adjacent  rail  and 
touching  the  dead  rail,  or  any  part  of  the  iron  work  of 
the  car,  will  get  a  severe  shock.  A  dead  rail  can  be  de- 
tected \vith  the  lamp  circuit.  Neither  the  car  lamps  nor 
the  test  lamps  will  light  on  the  dead  rail,  but  will  light  on 
the  rails  next  to  it.  To  have  a  car  move  itself  off  a  dead 
rail,  a  switch  iron  or  other  piece  of  metal  must  be  shoved 
down  between  the  dead  rail  and  the  one  next  to  it. 

GROUNDS  AND  SHORT  CIRCUITS. — If  a  car  fails  to 
start  on  the  first  notch  and  the  controller  flashes  when 
thrown  off,  it  may  be  due  to  any  of  several  causes :  It  is 
most  apt  to  be  due  to  a  ground,  short  circuit  or  wrong 
connection,  but  sometimes  it  may  be  due  to  stiffness  of 
the  bearings  or  sticking  of  the  brake-shoes.  With  the 
lamp  circuit,  the  existence  of  a  ground  can  be  quickly 
determined,  and  can  be  located  in  a  very  short  while. 
To  determine  if  a  ground  exists,  throw  the  reverse  switch 
to  the  "off"  position,  in  order  to  disconnect  the  field  that 
is  permanently  grounded,  and  touch  every  part  of  the 
controller  with  the  test  line.  No  part  of  the  controllers 
should  cause  the  lamps  to  light,  except  the  fingers  which 
are  permanently  grounded  to  the  upper  bar  of  the  right- 
hand  motor  cut-out.  If  the  test  shows  the  existence  of  a 
ground,  it  can  be  located  by  a  process  of  elimination. 
The  heater  and  lamp  switches  must  be  turned  off,  or  re- 
sults may  be  misleading.  A  ground  anywhere  from  L 
to  the  fuse-box  will  cause  a  demonstration  as  soon  as  the 


580  TESTING   OF   DYNAMOS   AND   MOTORS. 

pole  is  put  on  the  wire,  and  it  must  either  burn  itself  out 
or  blow  the  station  breaker,  because  the  car  fuse  can  not 
act.  If,  however,  breakers  are  used  instead  of  hood 
switches,  they  will  act  and  relieve  the  situation  promptly. 
If  a  ground  takes  place  bet  ween  the  fuse-box  and  the  trolley 
finger  on  either  controller,  the  car  fuse  will  blow  as  soon 
as  the  pole  is  put  on.  Such  grounds  generally  announce 
themselves  very  forcibly  and  do  enough  burning  to  make 
the  location  easy  to  find.  If  the  ground  is  between  the 
fuse-box  and  the  controller  trolley  finger,  and  it  can  not 
be  found  by  inspection,  proceed  as  follows :  Disconnect 
the  trolley  side  of  the  lightning  arrester  and  test  with  the 
lamp  circuit ;  if  the  ground  is  due  to  contact  between  the 
arrester  points,  the  disconnection  of  the  arrester  will  re- 
move the  ground.  If  it  does  not,  disconnect  the  blow 
coils  in  the  controllers,  one  at  a  time,  and  test  after  each 
disconnection.  If  the  disconnection  of  a  blow  coil  re- 
moves the  ground,  the  ground  is  in  that  coil.  If  it  is  in 
neither  blow  coil  nor  the  arrester,  it  must  be  in  one  of  the 
connecting  wires  or  in  the  main  car  trolley  wire. 

A  ground  in  the  starting  coil  or  any  of  its  connecting 
wires  or  fingers  will  cause  the  lamps  to  light  if  any  of 
the  wires  marked  Jt  or  19  are  touched  with  the  test  line, 
In  order  to  locate  the  affected  part  exactly,  the  wires 
must  be  disconnected,  one  at  a  time,  and  a  test  made  after 
each  disconnection.  As  soon  as  the  faulty  wire  is  dis- 
connected the  remaining  ones  will  not  light  the  test  lamps, 
but  the  faulty  one  will.  Grounds  on  the  resistances  are 
often  due  to  brake-rods,  levers,  chains,  etc.  A  ground 
on  motor  field  No.  i,  or  any  of  its  connections,  will  cause 
the  test  lamps  to  light  up,  when  the  test  line  is  touched 


CAR   EQUIPMENT  TESTS.  581 

to  any  connection  marked  /^  or  E^ ;  a  ground  on  the  No. 
i  motor  armature  will  show  up  the  lamps  on  any  A ,  or 
AAl  connection,  and  on  the  armature  itself.  To  tell 
whether  the  fault  is  in  the  armature  itself  or  in  some  wire 
leading  to  it,  draw  the  brushes  and  test  both  the  brush 
holders  and  the  commutator.  A  ground  on  motor  arma- 
ture No.  2  will  show  up  on  any  A2  or  A  A*  connection,  and 
must  be  located,  as  in  the  last  case.  Any  connection  marked 
F2  or  E2  will  always  show  up  a  ground,  because  one  end 
of  the  No.  2  field  is  permanently  connected  to  the  ground 
wire,  as  in  Fig.  199,  or  to  the  motor  frame,  so  that  in  or- 
der to  test  it  for  a  ground,  the  ground  wire  must  be  dis- 
connected. The  whole  principle  of  locating  a  ground 
lies  in  disconnecting  the  wires,  one  at  a  time,  and  elimi- 
nating the  faulty  one. 

It  is  possible  for  a  ground  to  exist  in  such  a  place, 
that  the  car  will  start  up  all  right  and  run  on  the  series 
notches,  but  will  blow  a  fuse  as  soon  as  the  motors  are 
thrown  to  multiple.  Take,  for  example,  a  ground  on  the 
motor  field  No.  i,  or  the  No.  2  motor  brush  holder.  In 
Fig.  202  it  can  be  seen  that  a  ground  at  G,  the  dotted  line, 
cuts  out  motor  No.  2  entirely ;  as  long  as  the  motors 
are  in  series,  the  current  passes  through  motor  No.  i 
and  to  the  ground,  through  the  fault.  As  soon  as  the 
motors  go  over  to  multiple,  the  fuse  will  blow  if  the 
ground  is  on  the  No.  2  armature  brush  holder,  but  there 
will  be  no  demonstration  at  all  if  the  ground  is  on  the 
negative  end  of  the  No.  i  field.  A  car  may  run  along 
for  days  in  this  condition  with  no  other  symptom  than 
that  the  car  starts  slowly,  and  runs  at  a  lower  speed  on 
series  points. 


582  TESTING   OF   DYNAMOS  AND   MOTORS. 

If  when  the  car  fails  to  start  the  controller  flashes  on 
being  thrown  off,  it  can  not  be  told  immediately  whether 
the  fault  is  due  to  a  ground,  short  circuit,  or  wrong  con- 
nection. To  tell  if  it  is  a  ground,  disconnect  the  motor 
ground  wire  and  try  the  controller  again ;  if  it  shows  no 
flash,  the  removal  of  the  ground  wire  has  removed  the 

T 


FIG.  202. 

only  ground,  so  that  the  fault  must  be  due  to  a  short  cir- 
cuit or  a  wrong  connection. 

Short  circuits  in  the  controller  or  in  the  car  wiring 
hose,  as  a  rule,  do  so  much  burning  that  they  are  easily 
located  by  the  eye.  A  short  circuit  between  controller 
parts  can  be  located  with  the  lamp  circuit,  as  has  been 
shown.  It  can  also  be  used  to  test  the  hose,  as  follows : 
The  hose  must  be  disconnected  at  both  ends;  the  resist- 
ance coil  and  both  motors  must  also  be  disconnected,  for, 
of  course,  if  the  wires  are  left  in  the  coil,  the  lamps  will 
light  from  one  wire  to  the  other  through  the  coil;  the  same 


CAR    EQUIPMENT  TESTS.  583 

is  true  in  regard  to  the  motor  wires.  The  test  lamp  con- 
nections of  Fig.  196  are  used;  one  test  line  is  hooked  on 
to  one  of  the  car  wires  and  the  other  test  line  used  to 
touch  every  other  car  wire.  If  the  tester  is  familiar  with 
the  controller  under  test,  it  is  unnecessary  to  disconnect 
the  controllers,  as  all  the  blocks  to  which  the  car  wires 
connect  are  insulated  from  each  other  when  both  drums 
are  at  the  "  off  "  position. 

There  are  two  very  common  sources  of  short  circuit 
that  care  must  be  taken  to  avoid.  The  trolley  and  ground 
wires  being  alike,  are  easily  confused;  if  they  become  in- 
terchanged in  either  controller,  the  car  fuse  blows  as  soon 
as  the  pole  is  put  on  the  wire,  or  as  seen  in  Fig.  199,  the 
current  passes  through  the  fuse-box  directly  to  the  ground. 
The  other  common  source  is  due  to  the  workmen's  practice 
of  using  a  wrench,  instead  of  the  handle,  to  turn  the  con- 
troller drums ;  the  result  is  that  a  reverse  drum  is  some- 
times left  at  an  "on"  position  on  one  of  the  controllers ; 
let  it  be  left  at  "go  ahead,"  for  example;  if  anyone  goes 
to  the  other  controller  and  throws  its  reverse  lever  to  "go 
ahead"  and  tries  to  start  the  car,  the  car  fuse  blows ;  be- 
cause the  two  controllers  arc  set  to  run  the  car  in  opposite 
directions,  with  the  result  that  there  is  a  short  circuit,  as 
soon  as  enough  of  the  starting  coil  is  cut  out. 

It  will  be  seen  in  Fig.  1 99  that  the  armature  wires  are 
crossed  in  one  controller ;  that  is,  the  A  l  armature  wire 
goes  to  the  A  l  connecting  board  block  on  the  front  con- 
troller, but  it  connects  to  the  A  A  l  block  on  the  rear  one; 
the  same  is  true  of  the  Xo.  2  armature  ;  this  is  done  so  that 
the  car  will  move  in  the  direction  that  the  reverse  lever 
points,  irrespective  of  which  controller  is  in  use.  The 
armature  wires  are  said  to  run  to  the  front  controller 


584  TESTING   OF  DYNAMOS  AND   MOTORS. 

straight,  and  to  the  rear  controller  crossed,  and  the  de- 
vice is  necessary  from  the  fact  that  the  controllers  face  op- 
positely, as  regards  the  direction  of  motion  of  the  car. 

It  has  been  shown  how  to  test  the  polarity  of  the  motors 
so  that  the  proper  direction  of  rotation  shall  be  insured. 
Experts,  as  a  rule,  do  not  resort  to  this  preliminary  test, 
preferring  to  connect  the  motors  up  "hit  or  miss,"  and 
correcting  the  job  where  a  trial  proves  it  necessary. 
This  "hit  or  miss"  method  of  connecting  can  result  in 
any  of  the  following  complications :  The  motors  may 
oppose  or  "buck"  each  other  on  one  or  both  ends  of  the 
car,  so  that  the  car  can  not  move  at  all  on  both  motors. 
The  car  may  move  oppositely  to  the  indication  of  the  re- 
verse lever,  on  one  or  both  ends.  One  motor 
may  obey  the  indication  of  the  reverse  switch 
on  both  ends  of  the  car,  and  the  other  motor 
disobey  it  on  one  or  both  ends.  Any  of  the  above  com- 
plications can  be  straightened  out  as  follows:  (i.)  If 
the  motors  buck  each  other,  the  gears  will  be  heard  to 
give  a  click  when  the  controller  is  put  on  the  first  notch, 
and  the  controller  will  flash  when  thrown  to  the  "off"  po- 
sition. To  settle  the  matter  conclusively  and  decide 
which  motor  must  be  corrected,  try  them  one  at  a  time, 
on  both  ends  of  the  car,  and  note  which  motor  obeys  the 
reverse  lever.  If  motor  No.  i  is  right  on  the  front 
end  and  wrong  on  the  rear  end,  while  motor  No.  2  is 
wrong  on  the  front  end  and  right  on  the  rear  end,  re- 
verse the  No.  2  armature  wires  in  the  front  controller,  and 
the  No.  i  armature  wires  in  the  rear  one.  If  the  No.  I 
motor  is  right  on  both  ends,  and  the  No.  2  wrong  on 
both  ends,  the  armature  leads  of  the  No.  2  motor  must 


CAR   EQUIPMENT   TESTS. 


585 


be  reversed  at  the  motor.  If  the  Xo.  I  motor  is  wrong 
on  botli  ends,  reverse  its  armature  leads  at  the  motor. 
(2.)  If  both  motors  are  right  on  one  end  and  buck  on  the 
other,  reverse  the  controller  armature  wires  on  the  motor 
that  is  wrong.  (3.)  If  the  car  moves  contrary  to  the 
indication  of  the  reverse  lever  on  one  end  and  is  all  right 
on  the  other,  reverse  the  controller  armature  wires  of 
T  T 


_D 


FIG.  203. 

both  motors  on  the  contrary  end.  If  the  movement  of 
the  car  is  contrary  on  both  ends,  reverse  both  motor  ar- 
mature leads  at  the  motors. 

Assuming  that  the  car  starts  properly  on  the  first 
notch,  it  should  notch  in  evenly  distributed  impulses  up 
to  the  last  series  notch,  where  the  loop  or  shunt  is  cut 
into  action.  If  any  of  the  resistance  wires  get  inter- 
changed in  a  controller,  it  will  cause  the  car  to  accelerate 
in  jumps  on  that  controller;  if  the  resistance  wires  get 
confused  at  the  coil  itself,  it  will  cause  bad  notching  at 


586  TESTING  OF  DYNAMOS  AND  MOTORS. 

both  ends.  For  example,  suppose  the  .^i  and  ^3  wires  get 
interchanged  at  the  coil  itself.  In  Fig.  203  view  a  shows 
the  resistance  wires  properly  connected;  view  b  shows 
the  J^l  and  ^3  wires  interchanged ;  in  view  b,  on  the  first 
notch,  the  current  takes  the  path,  T-T-L-M-N-O-P-G, 
to  the  ground;  the  starting  coil  is  cut  out  and  the  car 
starts  with  a  jump.  On  the  second  notch,  the  current 
path  is  T-T-L-B-D-P-G ;  half  the  coil  is  cut  in,  so  the 
car  runs  faster  on  the  first  notch  than  on  the  second.  On 
the  third  notch,  the  current  path  is  T-T-L-S-C-A-P-G ; 
the  whole  coil  is  cut  in  and  the  car  runs  slower  than  ever. 
Such  a  mistake  in  connections  is  most  noticeable  on 
grades  and  curves  and  with  loads,  the  car  jerking  and 
halting  in  a  very  disagreeable  manner.  The  controller 
tips  burn  and  blister,  and  the  starting  coil  heats.  Con- 
fusion of  the  Rz  and  ^3,  or  the  R^  and  Rl  wires  causes 
the  same  action,  but  to  a  less  degree.  If  the  car  starts 
smoothly  and  notches  evenly,  it  should  be  run  a  mile  or 
so  with  the  motor  armature  and  axle  cap  bolts  loosened 
up  a  little  to  give  the  bearings  a  chance  to  find  a  seat. 
In  starting  a  car  for  the  first  time,  the  whole  action  of  the 
car  is  more  or  less  stiff,  and  especially  is  this  true  of  the 
bearings.  It  is  a  good  idea  to  prime  the  bearings  with  a 
good  quality  of  cylinder  oil  to  start  with,  and  pack  the 
boxes  with  regular  motor  grease.  The  gear  cases  are 
filled  to  a  depth  of  about  6  in.  with  gear  grease.  If  none 
of  the  bearings  show  a  tendency  to  heat,  the  caps  can  be 
screwed  down  and  the  car  put  on  a  regular  run,  which, 
if  possible,  should  be  so  selected  as  to  have  the  car  pass 
the  shop,  the  first  day,  so  that  if  any  trouble  arises,  it  will 
be  easy  to  get  the  car  in.  If  any  bearing  shows  a  ten- 


CAR    EQUIPMENT   TESTS.  587 

dency  to  heat,  take  it  out  and  scrape  it  and  try  the  car 
again.  Under  no  circumstances  should  a  car  with  a 
faulty  bearing  be  turned  over  to  the  operating  company, 
as  it  is  liable  to  tie  up  the  road. 

This  completes  the  first  series  of  tests  on  a  car;  all  of 
them  can  be  made  with  the  lamp  circuit.  To  make  sonic 
of  the  tests  that  follow,  an  ammeter  or  voltmeter,  or  bo:h 
will  be  required. 

If  either  pair  of  car  wheels  shows  a  tendency  to  spin 
at  starting,  any  of  several  things  can  be  the  matter.  The 
brakes  may  be  poorly  adjusted,  so  that  when  released,  Un- 
shoes hug  one  pair  of  wheels ;  in  this  case,  it  is  the  wheels 
that  are  free  that  spin.  Xo  chronic  fault  to  which  cars 
are  heir  can  cause  more  trouble  than  sticking  brake- 
shoes.  There  is  a  constant  and  heavy  extra  load  imposed 
upon  the  motors.  This  causes  heating  of  the  motors, 
sparking,  blowing  of  fuses,  breaking  down  of  controllers, 
and  grounding  of  brush  holders.  When  the  shoes  stick, 
the  shoes  and  wheels  get  very  hot  and  give  off  an  odor 
that  is  easily  recognized.  If  the  wheels  start  to  spin  in 
passing  to  the  second  or  third  notch,  it  shows  that  the 
resistance  coil  is  not  divided  up  right ;  too  much  of  the 
coil  is  cut  out  at  a  time.  If  the  spinning  takes  place  on 
the  first  notch,  it  shows  the  entire  resistance  of  the  coil 
to  be  too  low.  A  two-section  coil  is  generally  divided,  so 
that  about  five-eighths  of  it  is  cut  out  on  the  second  notch 
and  three-eighths  on  the  third.  On  a  three-section  coil, 
half  of  the  coil  is  cut  out  on  the  second  notch,  quarter  on 
the  third,  and  the  remaining  quarter  on  the  fourth. 
Sometimes  a  car  will  spin  its  wheels  when  it  first  goes 
on  duty,  but  will  cease  to  as  soon  as  the  coil  heats  up. 


588  TESTING  OF  DYNAMOS  AND  MOTORS. 

A  very  simple  test  with  a  voltmeter  will  deter- 
mine whether  the  resistance  coil  is  properly  divided  or 
not.  The  car  is  allowed  to  run  along  on  the  first  notch, 
and  when  at  a  steady  speed,  the  drop  is  taken  on  the 
whole  coil,  and  on  each  section.  Suppose  that  the  drop 
over  all  this  is  130  volts  on  a  two-section  coil;  then  the 
drop  from  JKt  to  Rz  should  be  81  volts,  and  the  drop 
from  R%  to  RZ  ,  49  volts;  49 : 8 1  : :  3 :  5 ;  or  49  is  ^  of  1 30, 
and  8 1  is  f  of  130,  and  since  the  drops  are  proportional  to 
the  resistances  across  which  they  take  place,  the  resist- 
ances are  divided  in  the  same  ratio,  and  are  therefore 
right. 

Spinning  of  the  car  wheels  is  also  caused  by  a  short- 
circuited  or  wrongly  connected  loop,  or  baked  or  wrong- 
ly connected  field  coils.  Any  of  these  faults  can  be 
tested  for  with  a  voltmeter.  Suppose  the  loop  part  of 
the  field  is  short-circuited ;  as  this  part  of  the  field  is  not 
cut  out  till  the  fourth  notch  is  reached,  its  effects  can  be 
felt  on  all  notches  except  the  loop  notch.  If  the  loop 
wire  is  confused  with  one  of  the  end  field  wires,  one  part 
or  the  other  (according  to  which  end  field  wire  is  in  the 
confusion)  of  the  field  coil  is  cut  out  even  on  the  first 
notch,  with  practically  the  same  result  as  if  part  of  the 
field  were  short-circuited.  If  the  field  coils  are  baked  or 
wrongly  connected,  or  if  the  motor  case  is  partly  open, 
or  a  pole  piece  or  two  loose,  the  conditions  and  symptoms 
are  the  same  as  in  the  above  cases.  The  net  result  of  any 
of  these  faults  is  to  weaken  the  field  of  the  motor,  thereby 
decreasing  its  starting  power  and  increasing  the  speed. 
The  car  is  slow  in  starting  and  takes  more  time  to  get  under 
full  headway,  but  its  maximum  speed  is  greater  in  most 


CAR   EQUIPMENT   TESTS.  589 

cases  than  it  would  be  were  the  field  in  perfect  order.  A 
weak  field  on  one  motor  causes  the  motor  to  shirk  its 
load  on  the  series  notches,  and  to  take  more  than  its  share 
of  the  load  on  the  multiple  notches.  The  c.  e.  m.  f. 
of  a  motor,  being1  dependent  upon  the  field  strength,  is 
low  when  the  field  is  weak  ;  so  that  when  the  motors  are 
in  multiple,  and  each  has  an  independent  path,  tije  one 
with  the  weak  field  and  low  c.  e.  m.  f.  will  take  the  most 
current,  and  in  some  cases  the  faulty  motor  will  take  more 
current  than  both  ought  to  take  under  normal  conditions ; 
the  result  is,  the  car  blows  fuses  and  gives  general  trouble. 
When  the  motors  are  in  series,  the  c.  e.  m.  fs.  are  in 
series,  and  the  conditions  are  entirely  different.  Between 
the  line  and  the  ground,  the  line  e.  m.  f.  distributes 
itself  according  to  the  resistance  it  meets ;  where  the  re- 
sistance is  greatest,  the  greatest  drop  takes  place.  It  is 
upon  this  fact  that  is  based  a  voltmeter  test  for  determin- 
ing whether  or  not  the  motor  fields  are  unbalanced.  The 
test  is  conducted  as  follows :  The  controller  is  put  on  the 
third  notch,  and  the  car  allowed  to  get  under  full  head- 
way;  the  voltmeter  is  then,  by  means  of  test  lines,  ap- 
plied to  first  one  armature  and  then  the  other.  That  is, 
the  drop  is  taken  on  the  two  armatures,  so  that  they  may 
be  compared.  If  the  two  motors  were  perfectly  balanced, 
the  two  readings  would  be  the  same;  but  there  are  sev- 
eral factors  that  lend  an  influence  toward  making  a 
slight  difference  in  the  readings ;  among  them  are  the  fol- 
lowing: Slightly  different  shape  in  the  pole  pieces;  dif- 
ferent quality  of  steel  in  the  two  frames  or  cases ;  dif- 
ferent dimensions  of  cases  due  to  the  difference  in  shrink- 
age on  cooling;  difference  in  the  setting  of  the  brushes; 


590  TESTING   OF   DYNAMOS   AND    MOTORS. 

difference  in  the  size  of  the  car  wheels  to  which  the  two 
armatures  are  geared;  condition  of  the  commutators  in 
regard  to  size  and  cleanness  ;  condition  of  the  arma- 
ture bearings  in  regard  to  wear — the  mo^e  the  bearings 
are  worn,  the  wider  is  the  air  gap  on  top  and  the  nar- 
rower is  it  on  the  bottom;  the  internal  resistance  of  the 
two  motors  may  not  be  the  same.  All  of  these  minor  in- 
flnences  should  not  conspire  to  produce  a  total  differ- 
ence of  over  10  volts  in  the  two  readings. 

If  there  is  an  irregularity  in  the  fields  of  either  motor, 
it  may  cause  a  difference  of  anywhere  from  30  to  150 
volts  in  the  two  readings.  Let  us  suppose  that  the  line 
voltage  is  500  volts  and  that  the  drop  across  the  two 
armatures  at  uniform  speed  is,  respectively,  285  and  195 
volts,  a  difference  of  90  volts,  their  sum  being  480  volts ; 
the  20  volts  unaccounted  for  being  dropped  across  the  mo- 
tor fields  and  car  wires.  Where  there  is  a  difference  of  90 
volts  in  the  amount  of  line  pressure  absorbed  by  the  two 
motors  when  in  series,  there  is  something  radically  wrong 
with  the  motor  whose  armature  gives  the  lowest  reading. 
Its  field  is  abnormally  weak.  Inspection  will  prove  whether 
or  not  the  motor  case  is  partly  open  or  any  of  the  pole 
pieces  loose ;  if  these  parts  are  all  right,  the  trouble  must 
be  in  the  field  coils  themselves.  Suppose  that  the  loop 
wire  and  one  of  the  end  field  wires  of  one  motor  are  con- 
fused in  one  of  the  controllers;  in  this  case,  if  the  two 
motors  are  otherwise  all  right,  the  voltmeter  will  show  a 
discrepancy  in  their  c.  e.  m.  f.  only  on  one  end  of  the 
car;  if  the  loop  wire  and  field  wire  are  confused  at  the 
motor  itself,  the  discrepancy  in  e.  m.  f,  will  show  at  both 
ends.  The  voltmeter  will  tell  if  the  loop  wire  is  con- 


CAR   EQUIPMENT   TESTS.  59 1 

fused  with  either  field  wire;  apply  it  as  follows:  (Figs. 
199  and  1 86.  The  dotted  lines  in  Fig.  186  indicate  the 
loop  wire  and  F+  wire  to  be  interchanged ;  in  this  case 
only  the  upper  part  of  the  field  coil  is  in  use,  and  it  only 
takes  the  current.  The  result  is  that  when  the  volt  lines 
are  applied  to  the  Fl  and  E\ ,  and  the  F«  and  £»  field 
connecting  board  blocks,  the  drop  on  the  wrongly  con- 
nected field  will  be  only  about  half  what  it  is  on  the  good 
field.  To  prove  conclusively  if  the  loop  and  field  wire 
have  been  interchanged,  disconnect  the  loop  wire  from 
both  controllers;  on  the  controller  where  no  confusion 
exists  the  car  will  start  on  the  first  notch,  as  usual ;  but 
on  the  end  where  the  trouble  is,  the  car  will  not  start  un- 
til the  controller  reaches  the  loop  notch.  In  Fig.  186, 
loop  wire  L  and  field  wire  F-\-  have  exchanged  places, 
as  indicated  by  the  dotted  lines.  With  the  normal  con- 
nection the  current  path  on  the  first  position  is  T-FB-A- 
B-C-R-D-F+-O-F A+-A — N-G;  with  the  inter- 
changed connection,  the  current  path  becomes  T-FB-A- 
B-C-R-D-O-F — A+-A — .V-G;  the  loop  wire  has  be- 
come a  part  of  the  main  circuit,  and  disconnecting  it,  pre- 
vents the  car  from  starting  on  the  first  notch.  As  soon 
as  the  shoe,  Fig.  186  or  the  drum,  Fig.  199  reaches  the 
loop  position  (with  the  loop  wire  disconnected),  the  cur- 
rent passes  through  the  whole  field,  and  since  all  resist- 
ance is  cut  out,  the  car  starts  with  a  jump.  If  then,  the 
voltmeter  shows  half  the  normal  drop  between  the  field 
connecting  board  blocks,  shows  no  drop  at  all  between  the 
loop  block  and  one  of  the  field  blocks  (the  field  block  to 
which  the  loop  wire  has  been  run  by  mistake)  ;  and  the 
car  fails  to  start  until  the  loop  notch  is  reached,  the  evi- 


592  TESTING   OF   DYNAMOS   AND  MOTORS. 

dence  is  complete  enough  to  convict  the  loop  wire.  If 
the  symptoms  exist  on  one  controller,  the  trouble  is  in 
that  controller' s  wires ;  but  if  on  both  controllers,  the 
confusion  is  between  the  motors  and  the  car  wiring  hose. 
To  finally  clinch  the  diagnosis,  it  is  the  practice  to  dis- 
connect the  wires  involved  in  the  trouble  and  test  them 
out  with  the  test  lamps. 

If  the  connections  of  the  field  and  loop  prove  to  be 
all  right,  the  trouble  must  be  due  to  baked  or  short-cir- 
cuited or  wrongly  connected  field  coils,  inside  of  the  mo- 
tor. To  tell  if  the  coils  are  baked  or  short-circuited,  pro- 
ceed as  follows :  Get  a  field  coil  just  like  the  ones  in  the 
motors  and  set  it  inside  of  the  car ;  lift  the  trap  doors  and 
motor  covers,  so  that  the  motor  fields  may  acquire  about 
the  same  temperature  as  the  test  field  (the  motors  should 
be  allowed  to  cool  off  over  night).  Take  the  insula- 
tion off  the  field  connectors,  so  that  the  ends  of  the  field 
may  be  accessible  to  the  voltmeter  test  lines ;  connect  the 
test  field  in  series  with  one  of  the  motors ;  reverse  the  ar- 
mature terminals  on  one  motor  so  that  the  two  motors 
will  buck  each  other,  and  will,  therefore,  be  unable  to  start 
on  the  first  notch.  Everything  being  ready,  have  a  helper 
put  one  of  the  controllers  on  the  first  notch ;  quickly  take 
the  drop  on  the  two  motor  fields  and  test  field  and  throw 
the  controller  off  to  avoid  heating  the  starting  coil  too 
much.  Repeat  this  operation  several  times,  so  that  there 
may  be  several  sets  of  readings  to  compare.  -Each  read- 
ing in  the  last  set  will  be  lower  than  the  corresponding 
reading  in  the  first  set,  because  the  heating  of  the  start- 
ing coil  raises  its  resistance  rapidly  and  diminishes 
the  current;  but  the  relation  between  the  readings 


CAR   EQUIPMENT   TESTS.  593 

in  the  several  sets  should  remain  about  the  same.  Since 
eacli  motor  has  four  coils  in  it,  its  field  resistance 
should  be  four  times  as  great  as  that  of  the  test  coil 
and  the  drop,  therefore,  four  times  as  great.  If  the 
drop  on  one  of  the  motors  proves  to  be  only  three 
times  that  on  the  test  coil,  the  insulation  must  be 
skinned  off  the  back  field  connection  of  the  suspicious 
motor,  so  that  the  drop  can  be  taken  on  two  coils  at  a 
time.  The  current  is  turned  on  again  and  several  more 
sets  of  drops  taken  on  the  test  coil  and  the  motor.  The 
drop  on  each  pair  of  motor  coils  should  be  twice  that  on 
the  test  coil.  If  the  drop  on  one  pair  of  coils  is  twice  as 
much,  but  that  on  the  other  considerably  less,  it  indicates 
a  fault  local  to  the  low  drop  pair  of  coils ;  but  if  the  drop 
on  both  pairs  of  coils  is  low,  it  indicates  the  coils  to  be 
baked.  Acting  upon  this  indication,  the  best  thing  to  do 
is  to  lift  the  insulation  on  the  end  of  one  of  the  bottom 
coils  and  look  at  the  wire.  To  do  this,  the  motor  must 
be  opened.  The  bottom  coils  are  selected,  because,  being 
on  the  bottom  where  the  ventilation  is  poor,  they  are,  as 
a  rule,  the  first  to  bake.  If  the  coils  are  not  baked,  the 
fault  is  local  to  one  coil.  To  locate  this  coil,  take  the 
drop  on  all  four  of  the  coils,  one  at  a  time.  Single  coils 
are  sometimes  short-circuited  by  the  inside  end  touching 
one  of  the  top  layers  in  its  path  from  the  bottom  layer 
to  the  surface. 

On  equipments  that  permanently  ground  the  field  on 
motor  No.  2,  as  in  Fig.  199,  it  is  possible  for  a  second 
ground  to  develop  in  the  field  and  cut  out  a  coil  or  two. 
In  such  a  case,  the  voltmeter  will  show  no  drop  when  its 
lines  are  held  on  the  coils  on  the  ground  side  of  the  fault, 


594  TESTING   OF   DYNAMOS   AND   MOTORS. 

as  no  current  passes  through.  With  such  a  fault,  it  is 
possible  for  a  car  to  run  for  days,  chewing  up  commuta- 
tors, throwing  solder  in  armatures  and  grounding  brush 
holders. 

If  the  coils  prove  to  be  neither  baked  nor  short- 
circuited,  the  trouble  must  be  due  to  a  wrong  in- 
ternal field  connection.  One  coil  or  two  may  be  con- 
nected in  wrong,  thereby  reversing  them.  It  is  a  conserv- 
ative estimate  to  say  that  one-fourth  of  the  fuse  blowing 
and  field  roasting  troubles  encountered  on  modern  types  of 
motors  is  due  directly  or  indirectly  to  a  field  coil 
being  put  in  wrong  or  connected  wrong,  and,  in  modern 
motors,  all  the  blame  does  not  rest  on  the  man  that  puts 
in  the  coil.  In  the  effort  to  economize  on  space  inside 
of  the  motor,  and  to  facilitate  opening  the  motor  without 
disconnecting  any  of  the  wires,  the  path  given  to  them 
inside  of  the  motor  is  a  very  circuitous  one.  There  are 
points  where  the  field  connecting  wires  can  be  neither 
seen  nor  felt,  and  when  two  or  more  wires  disappear 
under  a  field  where  they  can  not  be  followed  with  the  eye 
or  hand,  it  is  impossible  to  tell  whether  they  are  con- 
nected right  or  not.  On  such  a  motor  it  is  the  practice 
of  the  pitmen  to  connect  a  new  field  in  just  as  the  one 
taken  out  was  connected.  If  a  motor  ever  happens  to 
leave  the  factory  with  a  wrongly  connected  field,  the 
chances  are  that  every  field  put  into  the  motor  for  some 
time  after  it  is  installed  will  be  put  in  the  same  way  and 
cause  trouble,  until  the  fault  is  detected  by  test.  If  one 
of  the  motors  on  a  car  has  a  reversed  coil,  the  following 
symptoms  prevail :  The  car  will  start  with  the  motors 
in  series,  even  though  the  faulty  motor  might  be  unable 


CAR    EQUIPMENT   TESTS.  595 

to  start  the  car  alone,  because  the  car  starts  on  the  good 
motor,  the  faulty  one  acting  simply  as  a  means  of  com- 
pleting the  circuit.  As  soon  as  the  motors  are  thrown 
over  to  the  multiple  and  each  has  an  independent  path  to 
earth,  the  faulty  motor,  having  a  very  low  c.  e.  in.  f., 
lets  in  a  current  that  blows  the  breaker  or  main  motor 
fuse. 

Where  the  car  is  too  heavily  fused,  or  the  breaker  is 
out  of  order,  a  car  will  run  along  with  a  reversed  field 
until  all  the  fields  begin  to  roast,  when  the  fuse  begins 
to  give  trouble.  A  reversed  coil  will  not  only  roast  itself, 
but  will  roast  all  coils  in  series  with  it,  so  that  when  one 
coil  in  a  motor  is  found  to  be  roasted,  all  of  them  had 
better  be  taken  out. 

A  reversed  coil  can  be  tested  for  by  means  of  a  compass 
or  wire  nail.  The  test  with  the  compass  docs  not  require 
that  the  motor  case  be  opened,  but  it  must  be  made  by  one 
familiar  with  the  tricks  of  compasses,  or  the  results  will 
be  misleading.  On  any  street  car  motor  the  poles  should 
alternate,  in  polarity ;  that  is,  if  any  given  pole  is  north, 
the  poles  adjacent  must  be  south,  and  vice  versa.  To 
test  with  a  compass,  a  current  must  be  sent  through  the 
motor  as  it  hangs  on  the  truck.  This  is  best  done 
by  using  an  outside  resistance — a  water  box  or  a  lot  of 
old  starting  coils — because  it  injures  the  regular  starting 
coil  to  subject  it  to  the  full  voltage  for  so  long  a  time. 
A  current  of  about  half  the  full  load  capacity  of  the  motor 
is  sent  through  the  field  alone.  Holding  the  compass  per- 
fectly level,  so  that  the  needle  will  not  stick,  it  is  passed 
entirely  around  the  motor  in  a  circle  whose  plane  is  per- 
pendicular to  the  axle  and  suspension  bar,  and  a  little  to 


596 


TESTING  OF  DYNAMOS  AND  MOTORS. 


one  side  of  the  center.  If  the  fields  are  connected  right 
the  needle  will  reverse  ends  every  time  a  pole  piece  is 
passed.  Where  two  adjacent  poles  are  alike,  and  there- 
fore of  the  wrong  relative  polarity,  the  needle  will  not 
reverse  ends. 

The  test  with  the  nail  is  along  the  same  lines  as  that  with 
the  compass,  but  as  it  is  made  on  the  inside  of  the  motor,  it 
requires  that  the  motor  be  opened,  the  armature  taken  out, 
and  the  case  closed  again.  This  is  necessary  from  the  fact 
that  the  space  inside  of  a  modern  motor  is  so  limited  that 


FIG.  205. 


FIG.  206. 


there  is  hardly  room  to  reach  all  parts  with  the  hand. 
Also,  when  a  current  passes  through  the  field  coils  and 
the  armature  is  in  place,  the  poles  induced  in  the  arma- 
ture confuse  the  tester  who  is  not  accustomed  to  make 
the  test.  The  test  is  especially  useful  in  shops  where 
whole  sets  of  fields  are  put  in  ;  also  at  depots,  to  test  a 
car  whose  record  leads  it  to  be  suspected  that  there  is 
something  wrong  with  the  field  connections.  The  test 
is  conducted  as  follows:  The  motor  case  is  opened,  the 
armature  taken  out  and  the  case  closed  again ;  a  current 
is  then  sent  through  the  field  coils  as  in  the  compass  test ; 


CAR   EQUIPMENT   TESTS.  597 

if  the  coils  are  connected  properly,  the  poles  will  alter- 
nate in  polarity  and  the  path  of  the  lines  of  force  will 
be  as  shown  in  Fig.  204  but  if  one  of  the  coils  is  of  the 
wrong-  polarity,  there  will  be  three  like  poles  adjacent  to 
each  other,  and  the  general  path  of  the  lines  of  force  will 
be  as  shown  by  the  dotted  lines  in  Fig.  206.  A  mag- 
netizable piece  of  metal,  if  free  to  move,  will  arrange 
itself  parallel  to  the  general  direction  of  the  lines  of  force 
of  the  field  that  it  is  in.  If  the  fields  are  right  and  the 
lines  flow  as  in  Fig.  204,  a  wire  nail  held  loosely  between 
the  thumb  and  forefinger  will,  when  passed  by  hand 
from  one  pole  piece  to  the  other,  take  an  easy,  natural 
path,  never  turning,  but  persevering  in  the  general  di- 
rection of  the  lines  of  force.  Its  pointed  end,  say,  is  the 
last  part  to  leave  the  one  pole  piece,  and  the  head  end  is 
the  first  part  to  touch  the  approaching  one.  If,  however, 
one  or  more  coils  are  reversed,  and  the  path  of  the  lines 
of  force  is  that  of  Figs.  205  or  206,  the  nail,  in  its  passage 
from  one  pole  piece  to  the  other,  will  show  a  tendency  to 
take  up  a  position  at  right  angles  to  the  general  path  of 
the  lines  of  force.  In  Fig.  204  a,  a,  a,  a,  show  the  posi- 
tions taken  by  the  nail  in  passing  from  one  pole  to  the 
other,  when  all  the  coils  are  connected  properly.  In  Fig. 
205,  the  right-hand  bottom  coil  is  wrong,  making  the 
pole  S  instead  of  N;  the  result  is  that  the  nail,  as  indi- 
cated at  a',  a',  takes  the  perpendicular  position  on  both 
sides  of  the  faulty  coil ;  whenever  a  single  coil  is  re- 
versed, then  the  action  of  the  nail  is  irregular  on  both 
sides  of  it ;  also  its  pole  piece  is  much  weaker  than  any 
of  the  others ;  the  pole  piece  opposite  being  the  strongest 
of  the  four.  To  right  matters,  it  is  only  necessary  to  re- 


598  TESTING   OF   DYNAMOS   AND    MOTORS. 

verse  the  connections  of  the  faulty  coil,  when  it  becomes 
an  N  pole,  and  the  condition  of  alternate  polarity  is  re- 
stored. 

In  Fig.  206,  two  coils  have  been  reversed,  with  the  re- 
sult that  the  top  and  bottom  halves  of  the  case  work 
against  each  other,  and  with  the  final  result  that  the  mo- 
tor, as  far  as  the  field  is  concerned,  becomes  a  bipolar 
motor  with  compound  pole  pieces.  It  can  not  act  as  a 
bipolar  motor,  however,  because  the  armature  is  not 
adapted  to  run  in  that  kind  of  a  field.  The  letters,  a,  a, 
and  a',  a',  show  the  positions  taken  up  by  the  nail ;  be- 
tween the  unlike  poles,  its  position  is  regular,  but  be- 
tween the  like  ones,  irregular,  so  that  when  the  nail  takes 
up  the  cross  wire  positions  at  opposite  ends  of  a  di- 
ameter, the  conclusion  is  in  order  that  two  of  the  field 
coils  are  connected  wrong,  and  the  diameter  is  the  di- 
viding line  between  the  two  halves,  each  of  which  has  a 
reversed  coil.  In  order  to  right  matters,  do  either  of 
two  things — reverse  the  connections  of  both  coils  in  either 
half,  or  just  have  these  two  coils  exchange  places.  It  is, 
of  course,  possible  for  the  coils  to  be  so  reversed  as  to  give 
the  polarity  shown  by  the  dotted  letters  in  Fig.  206.  In 
such  a  case,  the  irregularity  in  the  position  of  the  nail 
takes  place  on  the  dotted  vertical  diameter,  dividing  the 
field  into  a  right  and  left-hand  half,  instead  of  top  and 
bottom.  The  best  way  to  familiarize  oneself  with  the 
test  is  to  take  a  motor,  connect  the  fields  up  right  and 
wrong,  and  try  them.  After  the  "feel"  of  the  nail  is  once 
acquired,  there  is  nothing  difficult  in  the  test. 

A  two  field- coil  motor  with  one  coil  reversed,  or  a  four- 
coil  motor  with  two  coils  reversed,  will  not  start  a  car 


CAK    EQUIPMENT   TESTS. 


599 


alone  with  normal  current ;  upon  this  fact  is  based  a  very 
good  dynamic  test  for  determining  if  the  field  coils  on  a 
motor  are  reversed,  and  with  very  little  trouble.  The 
test  is  conducted  as  follows:  Disconnect  the  back  field 
connection,  thereby  dividing  the  motor  field  into  two 


halves  of  two  coils  each.  Regarding  each  half  as  if  it 
were  the  complete  field,  connect  the  two  halves  in,  one 
at  a  time,  and  try  to  start  the  car.  Suppose  that  the 
coils  in  the  top  half  are  connected  right  and  that  those 
in  the  bottom  half  oppose  each  other;  then,  when  the  top 
half  is  connected  in,  the  car  will  start  on  about  the  second 
notch ;  but  when  the  bottom  half  is  in,  the  car  will  not 
start  at  all,  because  the  two  oppositely  directed  pole 


6oo  TESTING  OF  DYNAMOS  AND  MOTORS. 

pieces  try  to  start  the  car  in  opposite  directions,  and  it 
can  not,  therefore,  start  at  all.  If  the  car  will  not  start 
or  spin  the  wheels  on  the  third  notch,  it  indicates  the  field 
coils  are  connected  wrong  in  both  halves.  This  test  is 
especially  valuable  in  that  it  can  be  conducted  by  anyone 
who  knows  how  to  connect  the  fields  and  start  the  car. 
Pitmen  can,  therefore,  use  it  for  determining  in  which 
half  of  the  case  the  faulty  field  lies,  and  thereby,  per- 
haps, save  labor  in  opening  up  the  motor. 

Fig.  207  (a),  (b)  and  (c),  show  two  field  coils  of  the 
general  shape  used  on  modern  motors.  There  are  four 
coils  to  a  motor,  and  the  coils  being  alike,  are  inter- 
changeable. This  is  a  good  feature,  in  that  only  one  style 
of  coil  need  be  kept  in  stock,  but  it  increases  the  liability 
of  confusing  connections.  Each  coil  has  an  inside  end, 
marked  7,  and  an  outside  end,  marked  O;  the  inside  ends 
can  be  generally  distinguished  by  the  fact  that,  as  a 
rule,  there  is  a  bump  on  the  coil  where  the  inside  end  is 
brought  to  the  surface.  In  the  motor,  an  inside  end  al- 
ways connects  to  an  inside  end,  and  an  outside  end  to  an 
outside  end,  leaving  two  inside  ends  or  two  outside  ends 
to  be  brought  out  of  the  motor  to  tap  on  to  the  car  wiring 
hose.  Fig.  207  (b),  shows  the  proper  connection  for  two 
fields ;  for  example,  suppose  that  these  two  fields  go  into 
the  top  half  of  the  motor ;  two  others,  similarly  connected, 
go  into  the  bottom  half;  the  second  pair  must  be  put  in 
so  that  similar  ends  of  the  two  pairs  shall  be  opposite; 
two  of  the  opposite  ends  connect  together,  and  the  two 
remaining  leads  are  brought  out  as  motor  leads.  There 
are  two  ways  in  which  two  coils  may  be  connected  to- 
gether wrongly.  Fig.  207  (a)  shows  one  way ;  here  one 


CAR    EQUIPMENT   TESTS.  6oi 

coil  has  been  turned  end  for  end.  Fig.  207  (c)  shows  a 
second  way ;  here  one  coil  has  been  turned  over  on  its 
back.  The  result  of  both  of  these  errors  is  to  bring  two 
adjacent  coils  together  in  such  a  way  that  the  inside  end 
of  one  connects  to  the  outside  end  of  the  other.  The  re- 
sult of  this  is  to  have  the  current  enter  and  leave  the  two 
coils  in  the  same  way,  thereby  making  their  polarities 
alike  when  they  ought  to  be  opposite.  If  the  current  goes 
into  one  coil  at  its  inside  end,  it  should  enter  the  coils 
next  to  it  on  either  side,  at  their  outside  ends.  There  is 
some  excuse  for  getting  a  coil  put  in  end  for  end,  for 
where  the  lugs  or  leads  are  not  marked  in  any  way,  and 
are  disposed  as  in  the  figure,  it  is  difficult  for  the  un- 
practiced  eye  to  distinguish  them.  There  is,  however, 
no  excuse  for  putting  a  coil  in  top  side  down,  as  the  one 
side  is  always  curved  to  fit  the  inside  shape  of  the  motor 
case,  and  to  get  the  flat  side  next  to  the  case,  the  coil 
has  to  be  forced.  The  only  absolutely  certain  way  to  in- 
sure that  the  coils  are  connected  right,  where  there  is  any 
doubt,  is  to  test  with  the  compass  or  nail. 

Where  an  armature  develops  an  actual  ground  or  short 
circuit,  or  open  circuit,  no  instrumental  test  is  necessary 
to  locate  it,  as  there  is  always  more  or  less  demonstra- 
tion. An  open-circuited  or  grounded  armature  can  not 
run  the  car  alone,  but  if  both  motors  are  cut  in  and  are 
in  series  the  good  motor  can  run  the  car,  unless  the 
ground  is  on  the  motor  next  to  the  trolley  wire,  in  which 
case  the  current  passes  to  the  ground  without  reaching 
the  good  motor  at  all.  When  the  motor  armature  next  the 
trolley  has  a  ground  the  action  of  the  armature,  when  the 
power  is  applied,  depends  upon  whether  the  field  is  next  to 


602  TESTING   OF   DYNAMOS   AND   MOTORS. 

the  ground,  as  in  Fig.  199,  or  whether  the  armature  is,  as 
is  the  case  on  some  wiring  diagrams.  If  the  field  is  next  to 
the  ground,  the  current  does  not  pass  through  it,  as  the 
fault  on  the  armature  cuts  it  out ;  so  the  main  motor  fuse 
blows  as  soon  as  the  controller  reaches  the  second  notch. 
If  the  armature  is  next  to  the  ground,  it  will,  when  the 
power  is  applied,  turn  partly  over,  far  enough  for  the 
ground  to  come  under  the  positive  brush  holder,  and 
stops.  If  the  controller  is  advanced  beyond  the  first  notch 
the  main  motor  fuse  blows. 

A  ground  on  the  motor  next  to  the  ground  will  per- 
mit the  car  to  run  on  the  good  motor  as  long  as  the  two 
are  in  series,  but  the  faulty  motor  will  run  with  a  jerky 
motion.  A  short  circuit  in  the  armature  winding  or  com- 
mutator will  cause  this  same  jerky  motion  of  the  arma- 
ture; but  the  two  faults  can  be  readily  distinguished  by 
their  action  when  the  ground  wire  is  disconnected.  If 
the.  fault  is  a  short-circuited  armature,  disconnecting 
the  car  ground  wire  will  open  the  circuit,  so  that  the 
car  can  not  start  on  either  motor ;  if  the  fault  is  a  ground, 
the  current  still  has  the  fault  through  which  to  pass  to 
earth  so  that  the  removal  of  the  car  ground  wire  does  not 
keep  the  car  from  starting  on  the  good  motor.  A  short- 
circuited  armature  can  be  readily  detected  by  means  of  a 
pocket  knife  or  a  piece  of  iron.  If  either  is  held  up 
near  the  head  of  the  armature  it  will,  if  the  armature 
has  a  short  circuit  in  it,  vibrate  or  pulsate  ;  on  modern 
types  of  armatures  the  slots  are  so  wide  that  they  will 
cause  some  pulsation  of  the  test  piece,  but  that  due  to 
short  circuit  is  readily  distinguished,  because  it  is  more 
violent  and  is  less  frequent  per  revolution  of  the  armature. 


CAR    EQUIPMENT   TESTS.  603 

The  first  symptom  of  an  open  circuit  in  an  arma- 
ture is  a  chewing  away  of  the  mica  from  between  the 
commutator  bars  to  which  the  open -circuited  coil  is  con- 
nected. This  takes  place  as  soon  as  the  open  circuit 
starts — while  there  is  still  a  contact,  but  a  poor  contact. 
As  soon  as  the  rupture  is  complete  a  ball  of  fire  follows 
the  commutator  around  when  the  armature  is  in  motion. 
A  motor  will  start  with  a  single  open  circuit  in  the  arma- 
ture, because  the  single  open  circuit  docs  not  open  the  ar- 
mature circuit  entirely,  there  being  two  paths  through  it. 
After  the  armature  is  in  motion,  both  halves  are  active  in 
turning  it,  because  one  half  is  intact,  and  the  other  half 
gets  current  through  the  arc  that  holds  across  the  break. 
An  armature  sometimes  gets  partially  open-circuited  all 
around  the  commutator,  due  to  the  fact  that  the  ex- 
cessive current,  due  to  abuse  of  some  kind,  has  melted 
the  solder  out  of  the  connections,  impairing  them. 

This  chapter  would  be  incomplete  were  no  allu- 
sion made  to  the  property  that  street  car  motors 
have  of  acting  as  electric  brakes  in  time  of  emergency. 
This  property  is  based  upon  one  or  two  properties  of 
dynamos,  and  to  understand  the  brake  action,  these  must 
be  briefly  outlined.  Two  dynamos  are  said  to  oppose  each 
other  when  they  are  in  scries,  as  far  as  connections  go, 
but  have  their  polarities  opposed,  i.  e.,  the  two  dynamos 
try  to  send  current  through  each  other  in  opposite  di- 
rections. In  Fig.  208,  ^,-f,  A , —  and  Fl  -f- ,  are  the 
armature  and  field  leads  of  one  dynamo;  At+t  A*  — 
and  F»  those  of  the  other.  The  connections  are  such 
that  machine  No.  i  tries  to  send  the  current  around 
the  circuit  clockwise,  while  No.  2  tries  to  send  it  counter 


604  TESTING  OF  DYNAMOS  AND   MOTORS. 

clockwise.  If  the  conditions  are  such  that  the  two  ma- 
chines have  the  same  e.  m.  f.,  no  current  can  flow,  be- 
cause the  e.  m.  f.  of  one  is  counteracted  by  the  equal  and 
opposite  e.  m.  f.  of  the  other.  If,  however,  for  any  rea- 
son, the  field  on  one  machine  becomes  stronger  than  that 
on  the  other,  the  machine  with  the  stronger  field 
will  force  a  current  back  through  the  one  with  the 
weaker  field  and  run  it  as  a  motor.  If  the  two  ma- 


'A1-  A2-N 

A1+ A  /  \    A     A2+ 


F2- 


FIG.  208. 

chines  have  no  field  save  that  due  to  their  residual 
magnetism,  the  machine  with  the  stronger  residual 
will  run  its  mate  as  a  motor  ;  before  this  can  happen, 
however,  one  other  condition  must  be  fulfilled.  The 
machines  shown  in  the  figure  are  series  machines ; 
street  car  motors  are  series  machines.  For  given 
connections,  series  machines  run  in  opposite  directions, 
as  dynamos  and  motors;  therefore,  a  street  car  motor 
mounted  on  a  car  under  headway  can  not  generate  unless 
either  its  field  or  its  armature  connections  are  reversed. 
Fig.  209  is  a  diagrammatic  sketch  of  the  general  con- 


CAR   EQUIPMENT   TESTS. 


605 


nections  of  two  motors  under  a  car,  the  motors  being  in 
series;  Fig.  2 10  is  the  same  for  the  two  motors  in  multiple. 
In  Fig.  210  as  long  as  the  car  is  taking  power,  the  current 
splits  at  ,r  and  divides  between  the  two  motors.  In  con- 
nection with  Fig.  210  suppose  that  the  car  is  going  at  a 
fair  rate  of  speed,  and  that  the  controller  is  thrown  to  the 

0T 


RESISTANCE 


RESISTANCE  J 

a 


X  d 


1       ' 

G 
FIG.  209.  FIG.  210. 

off  position,  and  the  reverse  lever  thrown  back ;  the 
throwing  of  the  reverse  switch  reverses  the  two  arma- 
tures, and  thereby  connects  the  two  motors  up  as  dyna- 
mos in  a  position  to  generate  as  soon  as  conditions  per- 
mit. The  condition  necessary  is  that  the  controller  be 
advanced  to  a  multiple  notch,  the  overhead  switch  being 
thrown,  to  cut  off  the  power  ;  the  result  of  this  is  to  throw 
the  two  motors  together  in  a  local  circuit,  as  shown  in 


606  TESTING   OF   DYNAMOS   AND   MOTORS. 

Fig.  210.  The  car  being  still  in  motion,  in  virtue  of  its 
momentum,  and  both  motors  being  connected  up  as 
dynamos,  each  machine  tries  to  act  as  a  generator  and 
run  its  mate  as  a  motor,  and  the  machine  that  has  the 
most  residual  magnetism  can  generate  the  higher  voltage, 
and  thereby  back  a  current  through  the  lower  voltage 
machine  and  run  its  armature  as  a  motor.  The  throwing 
of  the  reverse  lever  connects  both  motors  up  as  dy- 
namos, for  the  given  direction .  of  rotation,  as  has  been 
stated ;  so  that  as  soon  as  one  machine  becomes  a  motor 
with  this  connection,  the  wheels  to  which  it  is  geared, 
spin  around  in  the  opposite  direction  to  what  the  direc- 
tion of  motion  of  the  car  calls  for.  If  the  car  is  under 
good  headway,  when  the  motors  are  reversed,  as  soon  as 
one  of  them  takes  hold  as  a  dynamo,  the  speed  of  the  car 
gets  a  sudden  and  violent  check,  for  the  following  rea- 
sons :  In  the  first  place,  the  spinning  of  one  pair  of 
wheels  in  the  wrong  direction  has  a  retarding  effect. 
Also,  in  the  local  circuit  that  includes  the  two  dynamos, 
there  is  no  resistance  save  that  of  the  motors  themselves, 
so  that  the  machine  that  becomes  a  generator,  acts 
through  a  short  circuit  and  generates  a  very  large  cur- 
rent; this  gives  the  momentum  of  the  car  a  lot  of  work 
to  do,  and  consequently  checks  the  speed. 

It  is  a  good  thing  to  know  how  to  use  the  motors  as 
brakes,  for  there  is  no  reckoning  on  when  a  brake  chain 
or  rod  may  give  way  at  the  same  time  that  the  line  power 
fails,  leaving  the  car  helpless,  except  for  the  braking 
power  of  the  motors.  It  is  not  well,  however,  to  make  a 
practice  of  stopping  a  car  in  this  way,  for  since  the  re- 
sistance coil  and  fuse-box  are  outside  of  the  circuit  in 


CAR   EQUIPMENT   TESTS.  607 

which  the  dynamo  acts,  the  machines  are  not  protected 
from  overload,  save  by  the  spinning  of  the  wheels. 
Again,  the  sudden  reversal  of  one  armature  strains  its 
pinion  and  gear,  just  as  a  sudden  reversal  with  the  power 
does.  There  have  been  several  plans  devised  for  using 
the  generative  ability  of  the  motors  to  stop  the  car.  They 
all  depend  upon  the  use  of  a  resistance  to  keep  the  current 
and  braking  power  down  to  a  safe  value.  Most  of  these 
devices  require  that  the  hand  brake  be  used  to  bring  the 
car  to  a  stand,  or  to  hold  it  on  a  grade,  because  the  mo- 
tors, of  course,  cease  to  generate  as  soon  as  the  car  comes 
to  a  stop. 

To  stop  a  car.  then,  by  means  of  the  motors  and  an 
ordinary  controller,  when  the  car  is  moving  "ahead," 
throw  the  overhead  switch  off,  throw  the  reverse  lever 
back  (to  do  this  the  controller  must  be  thrown  to  the  "off" 
position),  and  advance  the  controller  to  the  multiple  po- 
sition. When  the  car  is  backing  up,  with  the  reverse  lever 
at  the  "back-up"  position,  throw  the  overhead  switch, 
throw  the  reverse  lever  "ahead,"  and  put  the  controller 
on  a  multiple  notch.  If,  however,  the  car  is  ascending 
a  grade,  and  the  loss  of  the  power  and  the  failure  of  the 
brake  rigging  starts  it  down  the  hill  backward,  do  not 
move  the  reverse  lever;  leave  it  where  it  is  and  simply  ad- 
vance the  controller  to  the  first  multiple  notch ;  as  far  as 
the  motors'  generating  is  concerned,  it  makes  no  differ- 
ence what  multiple  notch  the  controller  is  on,  because  the 
starting  coil  is  not  in  the  local  generating  circuit ;  but  if 
the  power  should  happen  to  come  back  on  the  line  while 
the  car  is  descending  the  hill,  it  will  check  the  speed  too 
suddenly,  if  all  resistance  is  cut  out,  will  blow  the 


608  TESTING   OF   DYNAMOS   AND   MOTORS. 

fuse,  and  perchance,  strip  a  pinion.  When  the  reverse 
lever  is  set  for  the  motors  to  generate,  it  may  take  sev- 
eral seconds  for  them  to  take  hold,  but  when  they  do, 
they  do  so  very  suddenly. 

One  other  strange  feature  about  street  car  mo- 
tors, is  the  peculiar  action  known  as  bucking,  which  is 
closely  related  to  their  braking  ability.  Bucking  takes 
place  most  commonly  on  equipments  that  have  the  motor 
armature  next  to  the  ground ;  why  this  is,  can  be  readily 
seen  in  Fig.  211,  where  T  is  the  trolley;  a,  the  motor  ar- 
mature ;  F,  the  field,  and  G  the  ground.  If  a  ground  oc- 
curs on  the  field  at  a'  for  example,  that  part  of  the  field 


FIG.  211. 


that  lies  between  the  fault  and  the  trolley  wire  becomes 
separately  excited ;  both  ends  of  the  armature  being 
grounded,  one  through  the  permanent  ground  and  the 
other  through  the  fault,  the  faulty  motor  runs  as  a  sep- 
arately excited  short-circuited  dynamo;  the  current  gen- 
erated by  the  armature  being  very  heavy,  the  drag  be- 
tween the  armature  and  pole  pieces  is  correspondingly 
so,  and  so  is  the  work  thrown  upon  the  car.  There  be- 
ing nothing  to  do  this  work  except  the  momentum  of  the 
car,  the  speed  receives  a  check  which  constitutes  the  evi- 
dence of  the  "buck."  To  aggravate  matters,  the  short 
circuiting  of  the  trolley  current  through  the  fault  robs 


CAR   EQUIPMENT  TESTS.  609 

the  car  of  all  line  power.  A  ground  at  a,  usually  on  the 
positive  brush  holder  of  the  motor,  separately  excites  the 
whole  field  and  makes  the  action  more  violent.  Very 
often  on  account  of  "crowding"  the  motor  beyond  its 
capacity,  the  current  will  jump  over  from  the  brush 
holder  to  the  motor  frame,  but  the  arc  does  not  hold ;  in 
such  a  case  the  car  gives  a  violent  kick,  and,  perhaps, 
blows  a  fuse ;  if  it  does  not  it  runs  on  as  usual  until  more 
abuse  is  heaped  on  it.  A  grounded  armature  on  a  ground 
return  circuit  will  cause  the  car  to  give  a  succession  of 
kicks,  until  it  is  brought  to  a  stand.  The  motor  will  have 
to  be  cut  out  before  the  car  can  be  run  to  the  house.  A 
short-circuited  armature  will  cause  a  car  to  buck  on  any 
kind  of  a  circuit,  the  violence  of  the  bucking  depending 
upon  the  gravity  of  the  short  circuit. 


APPENDIX. 


APPENDIX. 


TABLE  I. 
PROPERTIES  OF  COPPER  WIRE. 


RESISTANCES  PER 

co0! 

en    . 

£3 

z<      M 

WEIGHTS. 

i  ooo  FEET  IN 
INTERNATIONAL 

WaH 

H  C* 

•  «ij-/j~   i 

OHMS. 

1" 

5                                     Feet. 

Mile. 

At  60°  F. 

At  75°  F. 

0000 
000 

460.                 211  600. 

410.       I     168  too. 

64i. 
509- 

3382. 
2687. 

.048  ii 
.060  56 

.049  66 
.062  51 

00 

365.            133  225.       i     403. 

2   129. 

.076  42 

.078  87 

0 

325- 

105  625. 

320. 

i  688. 

•09639 

.09948 

I 

289. 

83521. 

253- 

'  335- 

.1219 

.125  8 

2 

258. 

66564. 

202. 

i  064. 

•152  9 

•'57  9 

3 

229. 

52441. 

'59- 

8^8. 

.194  i 

.2004 

4 

204. 

41  616. 

126. 

665. 

.244  6 

.252  5 

5 

182. 

33  124. 

100. 

529. 

•3°7  4 

•3'7  2 

6 

162. 

36  244. 

79- 

419. 

•3879 

.4004 

7 
8 

144. 
128. 

20  736. 

16  384. 

63. 
So. 

&: 

.491 
.621  4 

.506  7 
-64'  3 

9 

114. 

12  996. 

39- 

208. 

.7834 

.8085 

10 

103. 

10  404. 

32. 

1  66. 

•9785 

1.  01 

ii 

91. 

828l. 

25- 

132- 

1.229 

1.269 

12 

8t. 

656l. 

20. 

105. 

1.552 

i.  60  1 

»3 

72- 

5184. 

'5-7 

83- 

1.964 

2.027 

'4 

64. 

4  096. 

12.4 

65- 

2-485 

2.565 

II 

57- 
5'- 

3249- 
2  601  . 

9.8 
7-9 

52- 
42- 

3.133 
3-9U 

3-234 
4.04 

17 
18 

45- 
40. 

2  025. 
I  600. 

6.1 

4.8 

25^6 

5.028 
6.363 

5.189 
6.567 

ig 

36. 

1296. 

3-9 

20.7 

7-855 

8.108 

30 

I  024. 

16.4 

9.942 

10.26 

21 

28.5 

812.3 

,     a<5 

13- 

12-53 

12.94 

22 

25-3 

640., 

1.9 

10.2 

15.9 

16.41 

23 

22.6 

510.8 

1-5 

8.2 

19-93 

20.57 

24 

20.1 

404. 

1.2 

6-5 

25.2 

26.01 

25 
26 

17.9 

15-9 

320.4 
252.8 

•97 
•77 

4- 

3>-77 
40.27 

32-79 
41.56 

27 
28 

I4.2 
12.6 

201.6 

158.8 

.61 
•48 

3-2 

2.5 

50.49 
64-13 

52.11 
66.18 

29 

".3 

127.7 

•39 

2. 

79-73 

82.29 

30 

10. 

100. 

•3 

1.6 

101.8 

105.1 

3' 

8.9 

79  2 

•24 

1.27 

128.5 

132-7 

32 

8. 

64. 

.19 

1.03 

'59-1 

164.2 

33 

7.1 

50-4 

•IS 

.81 

202. 

208.4 

34 

6-3 

39-7 

.12 

.63 

256.5 

264.7 

P 

5-6 
5- 

3r-4 
25. 

2J 

•5 
•  4 

407.2 

335-1 
420.3 

614  APPENDIX. 

TABLE  II. 
TEMPERATURE  COEFFICIENTS. 

Table  of  Temperature  Variations  in  the  Resistance  of  Pure  Soft  Copper, 
according  to  Matthiessen's  Standard  and  Formulae. 


w 

p^  c/3  W 

Hg< 

TEMPERATURE 

gss 

COEFFICIENT 

OF 

LOGARITHM. 

INTERNATIONAL 
OHMS. 

«sg 

RESISTANCE. 

0 

I 

2. 

i. 

1.003  876 
1.007  764 

o. 

0.001  680  I 

0.003  358  8 

0.141  73 
0.142  28 
0.142  83 

3 

i.  01  1  66 

0.005  036  2 

o.  143  38 

4 

1.015  58 

0.006  712  i 

0.143  94 

5 

1-019  5 

0.008  386  4 

0.144  49 

6 

1.023  43 

o.oio  059  3 

0.14505 

7 

1.027  38 

o.on  730  7 

0.145  61 

8 

1-031  34 

0.013  4°°  3 

0.146  17 

9 

1-035  3i 

0.015  °68  3 

0.146  73 

10 

1.039  29 

0.016  734  6 

o.i47  3 

ii 

1.043  28 

0.018  399  3 

0.14786 

12 

1.047  28 

O.O2O  062  I 

0.148  43 

13 

1.051  29 

0.021   723 

0.149 

14 

1-055  32 

0.023  382  I 

0.149  57 

15 

J-o59  35 

0.025  039 

0.150  14 

16 

1.06339 

0.026  694 

0.150  71 

I7 

1.067  45 

O.O28  348 

0.151  29 

18 

1.071  52 

0.029  999  • 

0.151  86 

19 

1-°75  59 

0.031  644 

0.152  44 

20 

1.079  68 

0.033  294 

0.153  02 

21 

1.083  78 

0.034  939 

0.1536 

22 

1.087  88 

0.036  581 

0.154  18 

23 

1.092 

0.038  222 

o.i54  77 

24 

1.096  12 

0.039  859 

25 

1.  100  26 

0.041  494 

0.155  94 

26 

1.104  4 

0.043  I27 

0-15653 

27 

1.108  56 

0.044  758 

28 

1.  112  72 

0.046  385 

•157  7 

29 

1.116  89 

0.048  on 

0.158  3 

3° 

1.  121  07 

0.049  633 

0.158  89 

40 

£ 

1.163  32 
1.  206  25 
I  249  65 

0.065  699 
0.081  436 
0.096  787 

0.164  88 
0.17095 
0.177  n 

70 

1.293  27 

0.111  687 

0.183  29 

80 

1.336  81 

0.126  069 

0.183  46 

90 

1-37995 

0.139  863 

0.195  58 

TOO 

1.422  31 

0.152995 

0.201   58 

APPENDIX. 


6I.S 


TABLE  III. 
GALVANIZED  IRON  WIRE. 

(Taken  from  John  A.  Roebling's  Son's  Wire  in  Electrical  Construction.) 


WEIGHTS, 

RESISTANCE  PER  MILE 

POUNDS. 

IN  OHMS. 

HO 

$6 

Cfl 

is 

w  . 

H  — 

£C/3 

D    . 

<  55 

Zffl 

fc=Q 

5"" 

IOOO 

Feet. 

One 
Mile. 

Iron. 

Steel. 

00 
0 

I 

0 
I 
2 

340 
300 

284 

304 
237 
212 

I  607              2.93 
I  251              3.76 
I  121              4.19 

4-05 
5-2 

5-8 

2 

3 

259 

177 

932              5-04 

6.97 

3 

4 

238 

149 

787             5-97 

8.26 

5 

220 

127 

673             6.99 

9.66 

4                6 

203 

109 

573     ;         8.21 

11-35 

5 

7 

180     i 

85 

450           10.44 

M-43 

6 

8 

165 

72 

378            12.42 

17.18. 

7 

9 

148 

58 

305 

15  44 

21-35 

S 

10 

134 

47 

250 

18.83 

26.04 

8 

ii 

120      ! 

38 

200 

23.48 

3247 

9 

12 

109     | 

31 

I65 

28.46 

39.36 

ii 

13 

95 

24 

125 

37-47 

51.82 

12 

14 

83 

18 

96 

49-08 

67.88 

13 

15 

72 

13-7 

72 

65.23 

90.21 

16 

65 

n.  I 

59 

80.03 

110.7 

15 

17 

58 

8.9 

47 

100.5 

139- 

16 

18 

49 

6.3 

33 

140.8 

194.8 

6i6 


APPENDIX. 


TABLE  IV. 
CURRENT  CAPACITY  FOR  IRON  WIRE. 

(From  American  Electrician,  March,  1897.) 


NUMBER 
B.  &  S.  G. 

SAFE 
CURRENT 

IN 

WOOD 
FRAME. 

SAFE 
CURRENT 

IN 

IRON 
FRAME. 

SAFE 
CURRENT 

FOR 

ONE 
MINUTE. 

NUMBER 
OF  FEET 
PER  OHM. 

8 

17.4 

20.3 

43-6 

250 

9 

14.6 

17.1 

36.6 

i?3 

10 

12.3 

14-3 

30.8 

137 

ii 

10.3 

12 

25.8 

108 

12 

8.7 

10 

21.7 

86.4 

13 

7-3 

8.5 

18.3 

68.5 

14 

6.1 

7 

15-3 

54-3 

15 

5-i 

6 

12.9 

43-i 

16 

4-3 

5 

10.8 

34-1 

17 

3-6 

4.2 

9.1 

27.1 

18 

3 

3-5 

7.6 

24-3 

19 

2.52 

2-9 

6.3 

16.5 

20 

2.17 

2.5 

5-4 

13-5 

21 

1.82 

2.1 

4-5 

10.7 

22 

1-53 

1.77 

3-8 

8.49 

23 

1.28 

1.49 

3-2 

6.73 

24 

i.  08 

1.20 

2-3 

5-34 

APPENDIX. 


6i7 


TABLE  V. 

FUSING  EFFECTS  OF  CURRENTS. 

Table  giving  the  diameters  of  wires  of  various  materials  which  will   be 

fused  by  a  current  of  given  strength. 

W.  H.  PREECE,  F.  R.  S. 


d=  ( - 


i  d  =  diameter. 

-.  /  =  current  in  amperes. 

( a  =  constant. 


DIAMETERS  IN  INCHES. 

z. 

iS 

iT 

o> 

o' 

gg 

0 

1* 

c*  " 

iz£ 

•c  *" 

00 

V? 

•o  - 

s 

u 

8.3 

|tl 

X  "* 

c  || 

l\ 

1  II    ;     .  II 

II 

III 

•6 

§•* 

—  ^ 

es  * 

4> 

s  s         §  "         .E  * 

n  "3 

cs  3 

u 

* 

fc 

C 

o.       ^    i 

H 

f 

^ 

i 

0.002  I 

0.002  6 

0.0033 

0.0033 

0.003  5 

0.0047 

o  007  2    0.008  3 

0.008  i 

2 

0.003  4 

0.004  I 

0.0053 

0.005  } 

0.005  & 

0.007  4 

o.oi  i  3     0.013  2 

0.012  8 

3 

0.0044 

0.005  4 

0.007 

0.0069 

0.007  4 

0.009  7 

0.014  9    0.017  3 

0.016  8 

4 

0.0053 

0.006  5 

0.008  4 

0.008  4 

0.008  9 

0.011  7 

O.OlS  I    '  0.021 

0.020  3 

5 

0.006  2 

0.007  6 

0.009  8 

0.009  7 

o.oio  4     0.013  6 

0.021      i  0.024  3 

0.023  6 

10 

0.009  8 

0.012 

0.0155 

0.015  4 

0.016  4  i  0.021  6 

0.033  4 

0.038  6 

0.037  5 

15 

0.012  9 

0.015  8 

0.020  3 

O.O2O  2 

0.021   5 

0.028  3 

0.043  7 

0.0506 

0.049  x 

20 

'  0.015  6 

0.019  i 

0.024  6 

0.024  5 

0.026  i 

0.034  3 

0.052  9 

0.061  3 

0.059  5 

25 

0.018  i 

O.O22  2 

0.028  6 

0.028  4 

0.0303 

0.039  8 

0.061  4 

0.071  i 

0.069 

30 

\  0.020  5 

0.025 

0.032  3 

0.032           0.034  2 

0.045 

0.069  4 

0.080  3 

0.077  9 

35 

1  0.022  7 

0.0277 

0.035  8 

0.015  6 

0.037  9 

0.049  8 

0.076  9 

0.089 

0.0864 

4° 

0.024  8 

0.030  3 

0.039  * 

0.038  8 

0.041  4 

0.054  5 

0.084 

0.097  3 

0.0944 

45 

0.026  8 

0.032  8 

0.042  3 

0.042 

0.044  8 

0.058  9 

0.0909 

0.105  2 

0.102  I 

50 

0.028  8 

0.035  2 

0-045  4 

0.045 

0.048 

0.063  2 

0.097  5 

0.1129 

0.109  5 

60 

0.032  5 

0.039  7 

0.051  3 

0.050  9 

0.054  2 

0.071  4 

O.IIO  I 

0.127  5 

0.123  7 

70 

0.036 

0.044 

0.056  8 

0.056  4 

0.060  i 

0.079  » 

0.122 

0.141  3 

0.1371 

80 

0-039  4 

0.048  i 

0.062  i 

0.061  6 

0.065  7 

0.0864 

°-I334 

0.154  4 

0.1499 

9° 

0.042  6 

0.052 

0.067  2 

0.066  7 

0.071  i 

0.093  5 

O.J443 

0.167  I 

0.162  i 

100 

120 

0-045  7 
0.051  6 

0.055  8 
0.063 

0.072 

0.081  4 

0.071  5 
0.0808 

0.076  2 
0.086  I 

o.ioo  3 
0.113  3 

0.1548 
0.1748 

0.179  2 
O.2O2  4 

o.i739 
0.1964 

140 

0.057  2 

0.069  8 

O.OOX)  2 

0.0895 

0.0954 

0.125  5 

0.193  7 

0.224  3 

0.217  6 

160 
180 

0.062  5 
0.067  6 

0.0763 
0.082  6 

0.098  6 
o.  i  06  6 

0.097  8 
0.105  8 

0.1043 
0.112  8 

0.1372 
0.1484 

0.2II  8 

0.229  l 

0.245  2 
0.265  2 

0.2379 
0.257  3 

200 

0.072  5 

0.0886 

0.114  4 

0.1135 

o.iai 

0.1592 

0-245  7 

0.284  5 

0.276 

225 

0.078  4 

0.095  8 

'0.123  7 

O.  122  8 

0.1309 

O.I72  2 

0.265  8 

0.307  7 

0.2986 

250 

0.084  * 

O.  IO2  8 

0.132  7 

0.131  7 

0.1404 

0.184  8 

0.285  * 

0.3301 

0.3203 

275 

0.0897 

0.1095 

0.141  4 

0.1404 

0.149  7 

0.1969 

0.3038 

0.3518 

0.341  7 

300 

0.095 

0.   I   I'l    I 

0.149  8 

0.148  7 

0.1586 

0.208  6  1 

0.322 

0.3728 

0.361  7 

6i8 


APPENDIX. 


<D  y 

tf§ 

«  ?  "3 
2^3- 

B    K-  (B 


IP. 

g.0.0 


- 


<   O   ST. 

^    3 


3  en  S 
K-  "  o 
P  5^ 

?§  3 


O    OO^J  Ut  W 

«  o  ~j  £  w 

No.  B.  &  S.  G. 

00000 
00000 

o  S  -£  So  N 

00  OMO 

00000 

85i  a& 

00  ONUt  •*»•     H 

Diameter 
in  inches. 
d. 

0    O   0    0    0 

88888 
S%j?ti& 

M     H     M  <JJ  U) 

00000 

o  b  b  b  b 
|o\oo\K 

OsOOtn   H   o\ 

^b\-2^ 

fc. 

(fijee 

M  ^    O31JI  U) 

•fe-b1  £  ^ 

Copper, 
a  =  10  244. 

O    O  -^    OOUl 

^?&s 

Aluminum, 
a  =  7  585. 

O>vi  v£)   5  «5 

(0  OJ  (Jl    OO  M 
^.  tn  4>.  ^  ^) 

Platinum, 
a  —  5  172. 

Os<l  vo  W  -5 

(0  W  Ul    00  M 

German  Silver, 
a  =  5  230. 

ut  o  O  M  ON 

(0  W  Ln  ^J   O 
(0    10    O  <J    00 

Platinoid, 
a  =  4  750. 

*.  A    ON  00  O 

M    to  OJ  Ln  ^J 
Ol    M  OJ    M    IH 

Iron, 
a  =  3  148. 

K)   to  W  *•  tn 

OO  H  VI  V)   ^J 

Tin, 
a  =  i  642. 

M    10    10  U)  -^ 

o\o  i  S'o 

Tin-lead  Alloy, 
a  =  i  318. 

^M.w^ 

O\O  tn   K)   M 

Lead, 
a  =  i  379. 

00 

s 

ft) 

M 
O 

g  n 

O      > 

^  td 
o  r1 
a  w 


i 


APPENDIX. 

TABLE  VII. 
DYNAMO  TESTING  RECORD. 

Dynamo  Class Volts  no  load Volts  full  load... 

Machine  No Frame  No Spool  No 

Armature  No Rheostat  Type No 

Trimmings  Plate  No Location Date 


619 


^0 

?c 

^c 

Volts  Field. 
Ins.  No.... 

d* 
II 

1 

1 

n 

Remarks. 

COLD  TEST--Time             

7V  MI  f> 

S'o  load,  field  rheo.  all  out  
No  load    field  rheo  all  in        

Room 

|-...*C. 

No  load,  normal  voltage  

Full    load,    no    G.   S.  Shunt,  field 
rheo.  left  as  in  previous  readings 

Put  miscellaneous  readings  on 
back  of  this  sheet, 

COMPOUNDING  TEST  HOT. 
Time  

No  load  

i-ull         

iJo  load  

HOT  TEST—  Time 

Temp. 

'•      °C, 

No  load,  normal  voltage  

Room 

\ 

No  load,  field  rheo.  all  out  .. 

No  load,  field  rheo.  all  in  

Temperature °C.  after hours  run  at Volts Amps. 

Armature  •*  Surface Air  ducts Commutator 

(  End  connections Spider 

Spools,  by  increase  of  resistance .By  Therm Frame 

Resistance  by  bridge  ]  Shunt  Coil  Cold Hot .'. 

<  Series  Coil  Cold Hot 

Insulation  R.  of  machine Size  of  Shunt 

Insulation  R.  of  Spools Tested  by 

Insulation  R.  of  Armature. . . 


620 


APPENDIX. 


TABLE  VIII. 
DYNAMO  TESTING  RECORD. — Continued. 


REMARKS. 


MISCELLANEOUS  READINGS. 


Size  of  G.  S.  Shunt. 

Volts. 

Amp. 

Volts 
Field. 

Amp. 
Field. 

Speed. 

Pts. 
Rheo. 

• 

INDEX. 


Adding  tools  to  a  loaded  motor, 
496 

Alternating  current  circuits, 
grounds  on,  456,  457,  459 

Ammeter,  calibration  by  volt- 
ameter, 77;  construction  of 
shunt  for,  99;  graduation  of, 
93;  measuring  resistance  of 
grounds  by,  447;  several  in 
multiple,  100;  shunts  used 
with,  95 

Ampere,  basis  of,  70;  definition 
of,  14;  hour.  15;  international, 
17;  measurement  by  copper 
voltameter,  71 

Ampere's  principle,  20 

Arc  dynamos,  Brush,  260;  brush 
regulation,  238;  regulation, 
249;  in  series  and  multiple, 
247;  Thomson-Houston,  250; 
Westinghouse,  274 

Arc  lamps,  247 

Armature,  25,  26;  bar  to  bar 
test,  151;  cross  connecting, 
469;  drop  in,  281;  high  volt- 
age in  low  voltage  fields,  505; 
insulation  measurement,  194; 
location  of  faults,  149,  151;  lo- 
cation of  grounds,  152,  242, 
243;  "lost  volts,"  282;  reac- 
tion and  compounding,  315; 
reaction  in  series  machine, 233; 
resistance  and  efficiency,  284; 


rewinding,  237;  Siemens,  27, 
28;  Siemens  defect  of,  28; 
Siemens  E.  M.  F.  of,  28;  test 
for  open  and  short  circuit,  241 ; 
winding  and  temperature  limit 
295;  artificial  cooling,  296; 
grounds  in  street-car  type, 
6or;  open  circuits  in,  603;  size 
of  wire  on  street-car  type,  524; 
car  motor  type  of,  554 
Astatic  needle,  123. 

B 

Back  induction,  34 

Baked  field  coils,  556,  592 

Barlow's  wheel,  3,  4,  23 

Battery,  amalgamation,  120; 
Daniells  cell,  115;  efficiency, 
118;  E.  M.  F.,  119,  120;  in- 
ternal resistance,  119,  211; 
Leclanche  cell,  121;  maximum 
activity,  117;  maximum  cur- 
rent, 116;  maximum  current 
with  given  resistance,  118 

Brake  action  of  series  motors  in 
parallel,  603 

Brush  test,  433;  brush  regula- 
tion on  arc  dynamos,  238;  on 
shunt  dynamos  291 

Brush  arc  dynamo,  260;  arma- 
tures, 262;  brushes,  271;  com- 
mutator, 264,  272;  controller, 
266;  dial,  274;  E.  M.  F.  263; 
field  spools,  260;  regulator, 


621 


622 


INDEX. 


264,    268;     starting    up,    269; 
switches,    270;      troubles    of, 
272;  under  light  load,  271 
Bucking  of  car  motors,  608 
Burning  out  a  ground,  466 


Calomel  cell,  136 

Car  brake  shoes,  587 

Car  control,  modern  controller, 
545;  old  style  rheostat,  543; 
series  parallel,  545;  use  of  field 
shunts  and  loops,  542 

Car  controller,  abuse  of,  546; 
General  Electric  Go's.,  547; 
tests  of,  561 

Car  circuit  breaker,  518 

Car,  failure  to  start,  572 

Car  motors,  554;  lubrication  of, 
586 

Car,  starting  coils,  542;  starting 
up  first  time,  572 

Car  switches,  518 

Car  wheels,  spinning  of,  588 

Car  wiring  diagram,  573 

Car  wiring,  grounds  and  shorts, 
572;  locating  faults  in,  572, 
579;  tests  of,  572,  579;  wrong- 
ly connected,  584 

Cell  (see  battery) 

Centimeter,  6 

Choke  coil,  street-car  type,  533 

Circuit  breaker,  street-car  type, 
518;  cleaning  contacts  of,  522; 
testing  for  faults  in,  520 

Clark  cell,  136 

Commutator,  27;  insulation  of, 
200 

Compensating  coil  for  galvano- 
meter shunt,  98 

Compound-wound  machine,  303, 
314;  and  armature  reaction, 
315;  compounding  factors  in, 
308;  troubles  in,  400;  drop  on 
series  coils,  328 ;  German  silver 


shunt,  321,  323;  hit  and  miss 
method  of  compounding,  318, 
324;  limits  to,  316;  practice  of, 
318;  series  coils  on  large  and 
small  dynamos,  329;  and  speed 
323;  compounding  volts  per 
revolution,  324  (see  also  motor 
generator  test);  direction  of 
rotation  as  dynamo  and  motor, 
303;  introducing  into  circuit, 
335J  over  compounding, '  320, 
322;  running  in  multiple,  337, 
340;  running  in  series,  342 

Compound-wound  motor,  break- 
ing shunt  field  on,  494;  speed 
regulation,  492;  speed  with 
differential  connections,  492; 
and  shunt  board,  493,  494 

Conductivity,  specific,  61 

Controller, 'for  street  cars,  538, 
545 

Copper  voltameter,  directions  for 
use,  72;  formulae  for,  74;  plate 
form,  71;  spiral  coil  form,  75 

Core-loss  test,  419 

Coulomb,  15 

Counter  E.  M.  F.,  37 

Critical  speed  on  series  machine, 

303,  314 
Cross  connecting  armatures  and 

commutators,  469 
Cross  induction,  33 
Current,  definition  of,  14 


Dead  rails,  578 

Differential  galvanometer,  174 

Differential  connections,  tests 
for,  492 

Direction  of  rotation  dynamos 
and  motors,  300 

Distribution  test  for  E.  M.  F., 
Mordey's  method,  429;  Swin- 
burn's  method,  431;  Thomp- 
son's method,  432 


INDEX. 


6*3 


Dynamo  armature,  25,  26;  as 
motors,  54;  efficiency  test,  433; 
field  of,  24;  losses  in,  32;  of 
different  type  run  together, 

305 

Dynamometer,  Siemens,  104,106 
Dyne,  6 


Eddy  current  test,  423 

Edison  dynamo  field  connections, 
236 

Efficiency,  commercial,  35,  42; 
electrical,  34,  40;  at  maximum 
activity,  40;  maximum  com- 
mercial, 43 

Efficiency  test  of  dynamo,  433; 
of  motor,  487;  by  Prony  brake, 
488 

Electromagnetism,  18 

E.  M.  F.,  definition,  14,  29;  and 
potential  difference,  112;  and 
cost  of  field  winding,  283;  dis- 
tribution test,  429,  431,  432; 
and  motor  speed,  471;  low,  by 
opposed  dynamos,  507;  low,  by 
opposed  fields,  505,  506;  regu- 
lation on  shunt  machine,  282; 
sources  of,  114 

Energy,  4;  laws  of,  5 

Equalizing  bar,  theory  of,  331 ; 
running  without,  335:  on  com- 
pound-wound dynamos,  337; 
on  series  machines,  246 


F 


Faults  in  armatures,  149,  151 
Faults,  location  of,  in  car  wir- 
ing, 572,  579 

Field  coils,  car  motor  type,  555; 
locating  baked  coils,  592;  use 
of  shunt  and  loop,  542;  wrong- 
ly connected,  556,  594 


Field  connections  on  Edison  dy- 
namo, 236;  excitation  methods 
29,  30,  31 ;  excitation  voltage 
and  walls  consumed,  498;  sat- 
uration test,  426;  test  for  open 
and  short  circuit  in,  239;  re- 
sistance and  efficiency,  284; 
shunt,  watts  expended  in.  284 
Friction  losses  in  dynamo,  423 
Fuses,  copper  for  street  car 
work,  524;  deterioration  of. 
529;  proper  size  of,  524;  mag- 
netic blow-out  for,  527 


Galvanometer,  astatic  needle  for, 
123;  differential,  174;  direct- 
ing magnet,  124.  instrumental 
constant,  85;  insulation  for 
keys,  128;  proportion  box,  129, 
131;  proportion  lines,  128; 
range,  90;  range  varied  by  re- 
sistance box,  86;  shunt,  86; 
requisites  for,  122;  resistance 
boxes  and  moisture,  132;  set- 
ting up,  83,  88. 134;  shunt  and 
compensating  coil,  98;  shield 
of  iron  rings.  124;  taking  rapid 
readings,  127;  tangent,  prin- 
ciple of,  78;  wiring  of,  133 

Gears,  bushings  for,  569;  for 
street  car  motors,  567;  test- 
ing. 570 

Gram  unit  of  mass,  6 

Grounds  in  street  car  armatures, 
601;  in  street  car  wiring,  579 

Grounds,  detector  for  alternating 
circuits,  456;  telephone,  457; 
Stanley  static,  459;  lamps  and 
bell,  440:  delusive,  441;  volt- 
meter, 442;  methods  of  locat- 
ing, 460,  466;  measuring  re- 
sistance of,  ammeter  method, 
447,  452;  voltmeter  method, 
445,  453.  455 


624 


INDEX. 


II 


High  voltage  and  line  loss,  207 
Horse  power,  definition,  16 
Hysteresis,  423 

I 

Induction,  mutual  and  self,  23 
Insulation,  galvanometer  and 
grounds,  203,  205;  high  volt- 
age tests,  207;  of  long  lines, 
443;  measurement  of,  first 
method,  183;  second  method, 
185;  third  method,  of  insula- 
tors, 188;  fourth  method,  elec- 
trometer, 193;  fifth  method,  of 
armatures  by  voltmeter,  194; 
sixth  method, by  galvanometer, 
203;  bar  to  bar  test,  179;  of 
commutator,  200;  of  marine 
cables,  191;  of  underground 
cables.  192;  and  temperature, 
202;  and  voltage,  201;  test  in 
motor  generator  test,  409;  test 
cases,  portable,  210 

J 

Joule,  definition  of,  16 

K 

Kicking  coils,  for  use  with  car 
lightning  arresters,  533 


Lamp  bank,  care   in    handling, 

354 

Leclanch6  cell,  121 
Lightning,  nature  of  discharge, 

531 

Lightning  arresters,  531;  adjust- 
ment of  air  gap,  534,  538; 
choke  coils  for  use  with,  533; 
General  Electric  Go's,  type  of, 

534 
Line  loss  and  high  voltage,  207 


M 


Machines  of  different  type  run 
together,  305 

Magnetic  field,  8;  meridian  de- 
termination of,  125 

Magneto,  detecting,  and  locating 
grounds  by,  461 

Manganin,  219 

Matter,  4 

Motor-generator  tests,  brushes 
for,  386;  changing  over  with 
compound-wound  machines, 
372,  405;  compounding,  398; 
data,  hot,  404;  engine  as  loss 
supplier,  356;  field  connections 
380,  381;  field  polarity,  383, 
397;  full  test  with  details,  378; 
free  data,  396;  German  silver 
shunt,  404;  load  precipitated, 
reasons  for,  400;  load  failure 
to  go  on,  reason  for,  401; 
machines  of  different  current 
capacity,  366,  373,  374;  ma- 
chines of  different  current 
capacity  and  E.  M.  F.,  374; 
putting  on  load,  383,  385,  397; 
reversal  when  starting  up,  372; 
rocker  arm,  loose  effect  of, 
406;  series  field,  test  for,  388; 
sudden  removal  of  -load,  406, 
417;  voltage  to  start  multipolar 
motors,  387;  with  lamp  bank, 
3591  with  street  car-motors, 
481;  with  three  machines,  354, 
411 

Motors, belting  up  for  shop  work, 
495;  classification,  50,468;  dif- 
ferential, 53;  helping  over- 
loaded engine,  496-498;  prin- 
ciples of,  36;  proper  diameter 
for  pulley,  497;  separately  ex- 
cited, 50;  series,  51;  shunt,  50; 
speed,  49,  52, 471 ;  armature  for 
street  car  type,  554 ;  connecting 
up  on  car,  565;  field  coils  for 
street  car  type,  556;  grounds 


INDEX. 


62.5 


on,  602;  mounting  on  car,  570; 

street  car  types,   552;  tests  of 

street  car  type,  563;  trial  runs, 

566,  570 
Multi polar   motors   running    on 

two  brushes,  470 
Mutual  induction,  23 

O 

Oersted,  17 

Ohm,  13 

Ohm's  law,  15,  59;  and  C.  E.  M. 

F.,  64;  self-induction,  65 
Open    circuit,    locating,  114;   in 

armature,   149,   151,  241;  field, 

239 


Thomson's  slide  bridge,  166; 
high,  by  differential  galvanom- 
eter, 175,  179;  of  grounds  by 
ammeter,  447;  of  grounds  by 
voltmeter,  453;  Wheatstone's 
bridge,  156,  167;  box  bridge, 
160;  slide  bridge,  159;  inter- 
polation 173;  of  multiple  cir- 
cuits, 101,  102 

Resistance  specific,  221,  224; 
table,  223;  and  temperature 
coefficient,  220;  water  rheo- 
stat, 214 

Retentivity,  20 

Rheostats,  caution  in  using,  508 

Running  two  stations  in  multiple 
343 


Permeability,  10;  table,  12 

Pole  strength,  unit  of,  7 

Portable  test  cases,  210 

Prody  brake,  488 

Proportion  box  and  galvanom- 
eter, 129;  and  direct  reading 
scale,  131 

Proportion  lines  and  galvanom- 
eter, 128 


R 


Radiating  surface  and  weight, 
296 

Rating  of  dynamos  and  motors, 
508 

Residual  magnetism,  20 

Resistance,  13,  69,  145;  boxes 
and  moisture,  132;  liquid,  213; 
liquid  measurement;  by  tele- 
phone, 216;  measurement  of 
burning  lamp,  154;  method  of 
constant  deflection,  156;  by 
differential  galvanometer,  174; 
low,  by  comparison  of  poten- 
tial, 147;  low,  differential  gal- 
vanometer, 175,  179;  low, 
Vienna  method,  153;  low, 


Saturation  test,  426 

Self-exciting  dynamos,  trouble 
on,  234 

Self  induction.  23 

Separate  excitation  and  sparking 
507;  and  variation  of  E.  M.  F. 
with  current,  508 

Separately  excited  machine  di- 
rection of  rotation,  305 

Series  machines,  231;  arc  lamps 
run  in  multiple  and  series,  247; 
armature  reaction,  233;  critical 
speed,  232,  244:  direction  of 
rotation  as  dynamos  and  motor 
302;  flashing  at  commutator, 
233;  motor  tendency  to  race 
on  no  load,  476,  479;  proper 
speed  and  sparking,  245;  re- 
moval from  service,  244;  run 
in  multiple  and  series,  246,  348 

Series  motor  efficiency  test,  487; 
locating  faulty  motor,  490,  491 ; 
run  in  series,  500 
Shop  efficiency,  500 

Short  circuit  locating  in  arma- 
ture, 241;  in  field,  239 

Shunt,  theory  of,   81;  board   on 


626 


INDKX. 


compound-wound  motors,  493, 
494;  box,  83;  and  series  motors 
run  in  series,  500,  503;  stand- 
ard, 91,  92;  used  with  am- 
meters, 95 

Shunt  machine,  279;  direction  of 
rotation  as  dynamo  and  motor, 
300;  danger  of  reversal  when 
multiple,  300;  E.  M.  F.  regula- 
tion, 282;  by  brushes,  291; 
effect  of  adding  a  dynamo  to  a 
multiple  circuit,  310;  field  rheo- 
stat, 288,  297;  and  E.  M.  F., 
290,  292;  loss  in,  294;  test  for 
connections,  298,  299;  for  high 
voltage,  304;  motor  and  start- 
ing box,  472,  479;  on  short  cir- 
cuit, 288;  putting  into  multiple 
circuit,  306,  308;  putting  in 
multiple  circuit  to  be  watched, 
309;  run  in  multiple,  300;  series 
298;  and  series  motors,  speed 
on  removal  of  load,  480;  signs 
of  reversal,  312;  sparking  upon 
reversal,  313;  sudden  removal 
of  load,  289;  taking  from  mul- 
tiple circuit,  310,  311 

Shunt  winding  substituting  for 
series,  475,  476;  and  tempera- 
ture limit,  295 

Shunts  for  field  coils,  542 

Single  machine  compounding  on 
water  box,  375 

Slide  wire  bridge,  159 

Speed,  49 

Standard  cell,  Clark's  136; 
Daniell's  115 

Stanley  static  ground  detector 
459 

Starting  box  selection,  473 

Starting  coils,  construction  of, 
542;  continuity  tests,  560;  de- 
terioration of,  539;  mounting 
on  car,  542;  tests  of  insulation 
560  use  and  abuse  of,  538 


Storage    batteries     and    station 

efficiency,  347 
Switches,  car,  518 


Temperature  coefficient  for  cop- 
per, 217;  table  appendix;  by 
rise  of  resistance,  408;  and  in- 
sulation, 202;  limit  in  arma- 
ture and  field,  295 

Testing  low  voltage  machines, 
351;  machines  below  their  nor- 
mal voltage,  363 

Tests  of  street  car  wiring,  572, 

579 

Thomson-Houston  arc  dynamo, 
250;  air  blast,  254;  brushes, 
254;  causes  of  flashing,  253; 
cut-out,  258;  E.  M.  F.,  252; 
grounds  on  armature,  257; 
grounds  on  field,  256;  grounds 
on  regulator,  256;  induction 
due  to,  252;  lead  of  armature, 
258;  method  of  controlling 
sparking,  253;  regulator,  251; 
/'.anting  the  field,  258 
Torque,  definition  of,  44 
Trolley,  characteristics,  515;  de- 
scription of  parts,  511;  troubles 
with,  516;  lubrication  of  parts, 
513;  pole,  adjustment  of,  516; 
pole,  straigthening,  513;  stand, 
511;  spring,  517;  wheel,  bear- 
ings, 514;  wheel  life  of,  514; 
wheel,  tests  of  different  makes, 
515;  wheel,  wearing  of.  516 


U 


Underground   circuits,   grounds 

on,  462,  463 
Unipolar  motors,  468 
Units,  absolute  and  practical,  6; 

length,  mass,  time,  force,  6;  of 

pole  strength,  7 


INDEX. 


627 


Volt,  definition  of,  14 

Voltage,  regulation   by  opposed 
dynamos,  393 

Voltmeter,  calibration  of,  138; 
calibration  coil,  139;  calibra- 
tion box  method,  142;  Poggen 
dorf's  method,  140;  measuring 
current  by,  103;  measuring 
resistance  of  grounds  by,  445, 
453;  measuring  insulation  by, 
194;  use  of  multipliers  with, 
143 


W 

Watt,  15;  hour,  16;  meter,  109 
meter  recording,  no 

Westinghouse  arc  dynamo,  274; 
armature,  275;  commutator, 
277;  fields,  274,  275,  277;  reg- 
ulation,  275,  278 

Wheatstone's  bridge,  general  re- 
marks, 167;  interpolation,  173 
theory,  156 

Wiring,  street  car,  573 

Work,  Joule  unit  of,  16 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW 


AN     INITIAL     FINE     OF     25      CENTS 

WILL  BE  ASSESSED  FOR  FAILURE  TO  RETURN 
THIS  BOOK  ON  THE  DATE  DUE.  THE  PENALTY 
WILL  INCREASE  TO  SO  CENTS  ON  THE  FOURTH 
DAY  AND  TO  $1.OO  ON  THE  SEVENTH  DAY 
OVERDUE. 


ICLF 

IN) 

• 

Wf    28,99., 

FEB    24  1938 


OCT 


MAR  8  1941  M 
LIBRARY  USE 

SEP  15  ;ggg 


NOV2    J975X 


"038 


LD  21-50m-8,-32 


YC   19844 


